crystals

Article LED as Transmitter and Receiver of Light: A Simple Tool to Demonstration Photoelectric Effect

Giuseppe Schirripa Spagnolo 1, Fabio Leccese 2,* and Mariagrazia Leccisi 2

1 Dipartimento di Matematica e Fisica, Università degli Studi “Roma Tre”, 00154 Rome Italy; [email protected] 2 Dipartimento di Scienze, Università degli Studi “Roma Tre”, 00154 Roma, Italy; [email protected] * Correspondence: [email protected]



Received: 5 September 2019; Accepted: 9 October 2019; Published: 15 October 2019


Abstract: The experimental observations of the photoelectric effect show the properties of quantum mechanics of the electromagnetic field. For this reason, this important effect is commonly used as an introductory topic for the study of quantum physics. The “classical” demonstration of the photoelectric effect is very incisive; unfortunately, the experimental apparatus is not cheap and easy to realize. The typical use of LEDs is as light emitters, but they can be used even as photosensors and, in this case, they are sensitive to wavelengths equal to or shorter than the predominant wavelength it emits. Furthermore, a LED used as detector is sensitive to wavelengths equal to or shorter than the predominant wavelength it emits. This ability of LEDs offers the possibility of developing a simple tool able to demonstrate the photoelectric effect. This paper describes the realization of an economic, simple, easy and safe system to use for the experimental demonstration of the photoelectric effect, based on the LED to LED structure. The paper has educational purposes, oriented towards laboratory teaching activities.

Keywords: photoelectric effect; LED; Bi-directional communication; educational tool

1. Introduction The teaching on quantum mechanics and new technologies plays an important role in the academic community [1–3]. In particular, the role that quantum mechanics acts as a reference theory for the microscopic description of reality is generally recognized [4]. In school education the introduction of the corpuscular nature of light makes it possible to understand the performing of many commonly used technological objects. Richard Feynman emphasized the importance of this: “I want to emphasize that light comes in this form—particles. It is very important to know that light behaves like particles, especially for those of you who have gone to school, where you were probably told something about light behaving like waves” [5]. The photoelectric effect is generally used as the introductory topic for the study of quantum physics [6–10]. The main aspect of photoelectric effect is that showed that the kinetic energy of photoelectrons depends linearly on the incident light frequency but is independent of its intensity. This explanation is central in our understanding of light and it is demonstrated experimentally in many introductory physics courses. The classic apparatus for these demonstrations includes a phototube, a variable voltage source, current and voltage meters and a light source (a mercury lamp or an incandescent bulb) capable of generating several narrow bandwidths of light [11]. One drawback, to using “classical apparatus”, in student laboratory, is the light source. The incandescent source has inherent disadvantage of low intensity, mainly at the shorter wavelengths, while the mercury source can be dangerous (high temperature, ultraviolet output, mercury vapor). To solve these problems, Light Emitting Diodes (LEDs) can be used as light source [12,13].

Crystals 2019, 9, 531; doi:10.3390/cryst9100531 www.mdpi.com/journal/crystals CrystalsCrystals2019 2019, 9, ,9 531, x FOR PEER REVIEW 22 of of 17 17

LED technology is widely present in our everyday life, like car headlights, TV and monitor LED technology is widely present in our everyday life, like car headlights, TV and monitor technology, illumination systems for domestic and professional use [14–18]. In 2014 Isamu Akasaki, technology, illumination systems for domestic and professional use [14–18]. In 2014 Isamu Akasaki, Hiroshi Amano e Shuji Nakamura received the Nobel Prize in Physics for the development of a blue Hiroshi Amano e Shuji Nakamura received the Nobel Prize in Physics for the development of a blue light LED [19,20]. light LED [19,20]. Compared to traditional light sources, LEDs are devices with low-cost, very small dimensions, Compared to traditional light sources, LEDs are devices with low-cost, very small dimensions, high reliability, low heat generation, longer life, great availability in colors of the emitted light. The high reliability, low heat generation, longer life, great availability in colors of the emitted light. ability of LEDs to emit light at various wavelengths is well-known. Less familiar is the ability of LEDs The ability of LEDs to emit light at various wavelengths is well-known. Less familiar is the ability of to be used as selective light sensors. Indeed, the working principle of LED in the photodetector mode LEDs to be used as selective light sensors. Indeed, the working principle of LED in the photodetector is like that of a conventional photodiode [21–26]. mode is like that of a conventional photodiode [21–26]. The aim of this paper is to realize an economic, simple, easy and safe system to use for the The aim of this paper is to realize an economic, simple, easy and safe system to use for the experimental demonstration of the photoelectric effect. Tool also able to offer the possibility of experimental demonstration of the photoelectric effect. Tool also able to offer the possibility of experimenting with the use of LED not only as a light emitter but also as wavelength selective sensor. experimenting with the use of LED not only as a light emitter but also as wavelength selective sensor. The remaining of this paper is organized as follows. A brief description of the basic properties The remaining of this paper is organized as follows. A brief description of the basic properties of the photoelectric effect are described in section 2. Work's principle of LEDs is given in section 3. In of the photoelectric effect are described in Section2. Work’s principle of LEDs is given in Section3. section 4, we present the realized tool. Section 5 contains some notes on the possibility of making In Section4, we present the realized tool. Section5 contains some notes on the possibility of making wireless commissions using LED to LED systems. Finally, conclusions are drawn in section 6. wireless commissions using LED to LED systems. Finally, conclusions are drawn in Section6.

2.2. BasicBasic PropertiesProperties of of the the Photoelectric Photoelectric E Effectffect TheThe photoelectric photoelectric e ffeffectect occurs occurs when when a a light light beam beam is is incident incident on on a a metal metal surface surface causing causing electrons electrons toto be be ejected ejected from from the the surface. surface. One One or or more more of of the the outer outer electrons electrons of eachof each metal metal atom atom is free is free to wander to wander in thein fieldthe field of the of ion the cores, ion cores, effectively effectively binding binding the metal the together.metal together. In fact, In the fact, metal the crystal metal may crystal be viewedmay be asviewed a periodic as a arrangementperiodic arrangement of positively of positively charged ioncharged (the positivelyion (the positively charged nucleicharged and nuclei non-valence and non- electrons).valence electrons). The positive The ionspositive create ions a potential create a well,potential as shown well, as in shown the Figure in the1, and Figure the free1, and electrons the free occupyelectrons the occupy states ofthe lowest states energy. of lowest energy.

Figure 1. Electrons in a metal can be modelled as particles in a potential well. Highest lying electrons atFigure Energy 1. ΦElectrons(Fermi level)in a metal known can as be “work modelled function”. as particles in a potential well. Highest lying electrons at Energy Φ (Fermi level) known as “work function”. In a metallic crystal, the electrons fill the available states up to a certain energy called Fermi level. If an electronIn a metallic is given crystal, enough the electrons energy, fill it may the availabl be separatede states completely up to a certain from energy the metal called and Fermi wander level. freelyIf an outsideelectron ofis it.given The enough amount energy, of energy it may needed be toseparated “eject” thecompletely electron fromfrom thethe metalmetal isand called wander the workfreely function. outside of In it. other The words,amount the of workenergy function needed (toΦ )"eject" is the the amount electron of work from thatthe metal must beis called done onthe thework electron function. (located In other in the words, potential the work well) function to make (Φ it) free. is the The amount shallower of work the that well must (i.e., be the done lower on the the workelectron function (located “Φ ”),in the less potential is the energy well)required to make toit free. cause The the shallower emission the ofthe well electron. (i.e. the Sincelower dithefferent work metalfunction atoms “Φ have”), less di ffiserent the energy positive required ions disposition, to cause the it is em logicalission to of assume the electron. that the Since work different function metal (Φ) dependsatoms have on the different metal. positive ions disposition, it is logical to assume that the work function (Φ) depends on the metal.

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The photoelectric effect can be expressed by

Ek(max) = Elight −  (1) Crystals 2019, 9, 531 3 of 17 In Equation (1). Ek(max) is the maximum kinetic energy of the electrons ejected, Elight is the energy of the electromagnetic radiation incident on a metal surface, and Φ is the metal work function. TheThe basic photoelectric experimental effect set can up be which expressed explains by photoelectric effect is as given in Figure 2a. Typical setup consists of a metal plate on which light shines. This plate is called the emitter (sodium in figure). E = E Φ (1) Across from the emitter is a plate called collector.k(max) Thelight emitter− is connected to the negative terminal of a variable dc voltage source and the collector is connected to the positive terminal. In Equation (1). Ek(max) is the maximum kinetic energy of the electrons ejected, Elight is the energy of theLight electromagnetic strikes the emitter, radiation which incident causes on photoelect a metal surface,ric electrons and Φ tois thebe emitted. metal work The function. electrons are attractedThe to basic the experimentalpositively charged set up collector which explains and a photocurrent photoelectric eisff ectestabl isished. as given Generally, in Figure the2a. radiationTypical setup sent consistson the ofemitter a metal comes plate onfrom which a merc lightury shines. discharge This plate tube is that called produces the emitter light (sodium at the in characteristicfigure). Across frequencies from the of emitter the mercury is a plate spectrum. called collector. The individual The emitter mercury is connected lines can be to theselected negative by appropriateterminal of optical a variable filters. dc voltage source and the collector is connected to the positive terminal.

(a) (b)

FigureFigure 2. 2. TheThe Photoelectric Photoelectric Effect Effect involves involves the the irradiatin irradiatingg a ametal metal surface surface with with photons photons of of sufficiently sufficiently highhigh energy energy to to causes causes electrons electrons to to be be ejected ejected from from the the metal. metal. Sodium Sodium work work function function 2,75 2.75 eV eV [27,28]. [27,28 ]. (a)(a )shows shows a asketch sketch of of a a photoelectric photoelectric test test device. device. (b) (b )shows shows the the typical typical imagine imagine used used to to explain explain the the photoelectricphotoelectric effect. effect.

AccordingLight strikes to the the classical emitter, whichwave theory causes of photoelectric light, the magnitude electrons toof bethe emitted. electric Thefield electronsvector ofare a lightattracted wave to goes the like positively the square charged root collector of the intensity and a photocurrent of the light, isElight established.∝ Ilight.Generally, Thus, as the the intensity radiation increases,sent on the the emitter electric comesfield magnitude from a mercury increases. discharge Since the tube force that on produces an electron light is proportional at the characteristic to the electricfrequencies field vector, of the mercuryit is expected spectrum. that the The kineti individualc energy mercury of a photoelectron lines can be should selected increase by appropriate with the intensityoptical filters. of incident light. Furthermore, the photoemission should occur at all wavelengths of incidentAccording light, if the to the incident classical radiation wave theory has enough of light, intensity. the magnitude of the electric field vector of a light p waveIn goescontrast, like the the experiment square root showed of the intensitythat the ma ofximum the light, speedElight of photoelectronsIlight. Thus, does as thenot intensitydepend ∝ onincreases, intensity; the the electric light fieldintensity magnitude affects increases.only the number Since the of force ejected on anelectrons electron and is proportional not their kinetic to the energies.electric fieldIn particular, vector, it isthe expected maximum that of the kinetic kinetic ener energygy of ofthe a photoelectronemitted electron should be contingent increase withon the the typeintensity of emitter of incident used and, light. on Furthermore, the color (wav theelength) photoemission of the incident should occurlight radiation. at all wavelengths of incident light,Einstein if the incident [9], to resolve radiation the hascontradictions enough intensity. between the classical theory of electromagnetic waves and theIn experimental contrast, the experiment results of the showed photoelectric that the maximumeffect, the speedenergy of carried photoelectrons by each particle does not of depend light (calledon intensity; quanta the or lightphoton) intensity is depe affndentects only on thethe numberlight’s frequency of ejected electrons(). In other and words, not their the kinetic photons energies. of a lightIn particular, beam have the a characteristic maximum of energy kinetic proporti energyonal of the to emittedthe frequency electron of bethe contingentlight. That onis: the type of emitter used and, on the color (wavelength) of the incident light radiation. E = h·ν (h = Planck’s constant = 6.6261·10-34 J·s (2) Einstein [9], to resolve the contradictions between the classical theory of electromagnetic waves andEinstein the experimental theorized resultsthat the of light the is photoelectric bundled up eintoffect, photons. the energy When carried a photon by each falls particleon the surface of light of(called a metal, quanta all the or energy photon) of the is dependent photon can on be the transf light’serred frequency to the electron. (ν). In A other part words,of this energy the photons is used of a light beam have a characteristic energy proportional to the frequency of the light. That is:

34 E = h ν (h = Planck0s constant = 6.6261 10− J s) (2) · · · Crystals 2019, 9, 531 4 of 17

Einstein theorized that the light is bundled up into photons. When a photon falls on the surface of a metal, all the energy of the photon can be transferred to the electron. A part of this energy is used to Crystals 2019, 9, x FOR PEER REVIEW 4 of 17 remove the electron from the metal atom’s grasp (Φ) and the rest is given to the ejected electron as kineticto remove energy. the electron Electrons from emitted the metal from atom’s underneath grasp the() metaland the surface rest is losegiven some to the of ejected the kinetic electron energy as duringkinetic theenergy. collision. Electrons But theemitted surface from electrons underneath carry allthe the metal kinetic surface energy lose imparted some of bythe the kinetic photon energy and haveduring the the maximum collision. kineticBut the energy. surface electrons carry all the kinetic energy imparted by the photon and haveWe the mustmaximum also consider kinetic energy. that the electrons can absorb energy from photons according to the rule “all orWe nothing” must also principle. consider All that the the energy electrons from can one ab photonsorb energy must be from absorbed photons and according used to liberate to the rule one electron"all or nothing" from atomic principle. binding, All the or elseenergy the from energy one is photon re-emitted. must be absorbed and used to liberate one electronWith from Einstein’s atomic interpretation, binding, or else Equation the energy (1) becomes:is re-emitted. With Einstein's interpretation, Equation 1 becomes: E = h ν Φ = (h c/λ) Φ (3) k (max) · − · − Ek (max) = h·ν −  = (h·c ⁄) −  (3) where c is the light speed and λ le wavelength of the incident light. where c is the light speed and λ le wavelength of the incident light. The Equation (3) shows that, given the work function (Φ) (chosen the metal on which to make the The Equation (3) shows that, given the work function (Φ) (chosen the metal on which to make light engrave), there exists a certain minimum frequency of incident radiation (ν = Φ/h) below which the light engrave), there exists a certain minimum frequency of incident radiation (=/ℎ) below no photoelectrons are emitted; this represents one of the evidences that the energy carried by light which no photoelectrons are emitted; this represents one of the evidences that the energy carried by is quantized. light is quantized. What described above represents the external photoelectric effect. In the external photoelectric What described above represents the external photoelectric effect. In the external photoelectric effect, the electrons present on the surface of a metal receive energy from the absorption of a photon. effect, the electrons present on the surface of a metal receive energy from the absorption of a photon. If the energy absorbed is greater than the binding energy (work function Φ), they acquire kinetic energy If the energy absorbed is greater than the binding energy (work function Φ), they acquire kinetic which allows them to be able to leave the metal. energy which allows them to be able to leave the metal. Electrons in an isolated atom can only have discrete energy levels, but when atoms are together as Electrons in an isolated atom can only have discrete energy levels, but when atoms are together in crystalline solids, these energy levels will split into many separated. Because the levels are so closely as in crystalline solids, these energy levels will split into many separated. Because the levels are so separated, they may be treated as a continuous band of allowed energy states. Therefore, in solid-state closely separated, they may be treated as a continuous band of allowed energy states. Therefore, in physics the electronic band structure of a crystalline solids describes the energy states that an electron solid-state physics the electronic band structure of a crystalline solids describes the energy states that is allowed or forbidden to be in. Most of the states with low energy (closer to the nucleus) are occupied, an electron is allowed or forbidden to be in. Most of the states with low energy (closer to the nucleus) up to a particular band; usually, the highest filled band is called Valence Band (VB), whereas the lowest are occupied, up to a particular band; usually, the highest filled band is called Valence Band (VB), empty band is termed Conduction Band (CB) (see Figure3)[29]. whereas the lowest empty band is termed Conduction Band (CB) (see Figure 3) [29].

Figure 3. Electronic band structure of solids.

In general,general, band band structures structures are are complex. complex. It is It possible is possible to find to di fffinderent different bands that bands are notthat separated are not byseparated energy gapsby energy but can gaps overlap but can instead. overlap Semiconductors instead. Semiconductors and insulators and are insulators distinguished are distinguished from metals byfrom the metals population by the of population electrons in of each electrons band. in In each each band. metal In there each is metal no band there gap is betweenno band gap their between valence andtheir conduction valence and bands. conduction In the bands. next section, In the morenext se detailsction, onmore the details energy on bands the energy formation bands will formation be given. will beDue given. to the band structure of the energy levels of the electrons in a crystalline solid, there is the possibilityDue to of the observing band structure the internal of the photoelectricenergy levels eofff ect.the electrons This effect in occursa crystalline when solid, the electrons there is getthe possibility of observing the internal photoelectric effect. This effect occurs when the electrons get energy from the absorption of a photon. Due to this increase in energy, the electron is promoted from the valence band into the conduction band. It is not ejected from the material surface but is internally able to flow forming a photo-current or is able to modify the material conductivity.

Crystals 2019, 9, 531 5 of 17 energy from the absorption of a photon. Due to this increase in energy, the electron is promoted from the valence band into the conduction band. It is not ejected from the material surface but is internally ableCrystals to flow2019, 9 forming, x FOR PEER a photo-current REVIEW or is able to modify the material conductivity. 5 of 17 Figure4 shows schematically the di fference between the external and the internal photoelectricFigure 4 e ffshowsect. schematically the difference between the external and the internal photoelectric effect.

Figure 4. Difference between the external and the internal photoelectric effect. Figure 4. Difference between the external and the internal photoelectric effect. 3. Photoelectric Effect in LED 3. Photoelectric Effect in LED The experimental apparatus of Figure2a is well suitable for quantitative study of the photoelectric effect,The but theexperimental apparatus isapparatus too complex of forFigure demonstration 2a is well in suitable undergraduate for quantitative classrooms. study of the photoelectricTo realize aneffect, economic, but the simple, apparatus easy and is safe too system complex to use for for thedemonstration experimental in demonstration undergraduate of theclassrooms. photoelectric effect, it is possible to use a LEDs-LEDs system. In other words, a series of LEDs is used toTo vary realize the an wavelength economic, of simple, the incident easy and radiation. safe system Additionally, to use for other the experimental LEDS are used demonstration as materials onof whichthe photoelectric the light is senteffect, (LEDs it is possible used as selectiveto use a LEDs-LEDs wavelength system. sensors). In other words, a series of LEDs is usedCompared to vary to the the “classic”wavelength apparatus, of the thisincident new systemradiation. replaces Additionally, the mercury other lamp, LEDS combined are used with as wavelengthmaterials on selection which the filters, light with is sent LEDs (LEDs [12 ,used30]. Furthermore,as selective wavelength other LEDs sensors). are used as light receivers; they replaceCompared the metalsto the "classic" on which apparatus, electromagnetic this new radiation system replaces is sent [31 the–34 mercury]. lamp, combined with wavelength selection filters, with LEDs [12,30]. Furthermore, other LEDs are used as light receivers; 3.1.they Semiconductor replace the metals LEDs, on How which Do They elec Work?tromagnetic radiation is sent [31–34]. The working principle of the Light Emitting Diode (LED) is based on the quantum theory. 3.1. Semiconductor LEDs, How Do They Work? A particle, as can be considered the photon and the electron according to Einstein’s special relativityThe theoryworking [35 principle,36], has energy:of the Light Emitting Diode (LED) is based on the quantum theory. A particle, as can be considered the photon and the electron according to Einstein’s special 2 relativity theory [35,36], hasE energy:2 = (m c2) + p2 c2 (for photon : m = 0) (4) 0· · 0 = ( ) + ∙ (for photon: =0) (4) Here: E is energy; m0 is rest mass; c is vacuum speed of light; p is momentum. Therefore, for photonHere: (E = h νis) weenergy; have: m is rest mass; c is vacuum speed of light; p is momentum. Therefore, for · 0 photon (E = h⋅ν) we have: p = E/c = hν/c = h/λ (5) Introducing the wave number k = 2=π/λ we⁄ =ℎ=ℎ can write:⁄ ⁄ (5) =2⁄ Introducing the wave number p = weh/λ can= write:h k (6) · p = h⁄ λ = ħ k (6) In Equation (5),h ¯ represent the Dirac constant [h= h/(2π)]. ⁄ AtomsIn Equation of solid-state (5), ħ represent materials the have Dirac a su constantfficiently strongħ=h (2 interactionπ) . that they cannot be treated as individualAtoms entities. of solid-state Valence materials electrons have are not a sufficiently attached (bound) strong to interaction individual that atoms; they rather, cannot they be belongtreated toas the individual system of entities. atoms. Valence electrons are not attached (bound) to individual atoms; rather, they belong to the system of atoms. The band structure of a crystalline solid is the relationship between the energy E of the electrons of the solid and the wave vector k (i.e. momentum) [37]. It comes from the Schrödinger equation (Equation (7)) of an approximate one-electron problem in a periodic potential U(r) with the periodicity of the crystal lattice:

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The band structure of a crystalline solid is the relationship between the energy E of the electrons of the solid and the wave vector k (i.e., momentum) [37]. It comes from the Schrödinger equation (Equation (7)) of an approximate one-electron problem in a periodic potential U(r) with the periodicity Crystals 2019, 9, x FOR PEER REVIEW 6 of 17 of the crystal lattice: " 2 # h 2 2 +U(r) Ψ (r) = E Ψ (r) (7) −2mħ∇ · k k· k - ∇2+U(r) ∙  (r) = E · (r) (7) 2m k k k In Equation (7), h¯ represent the Dirac constant [h= h/(2π)], r is a vector of the direct lattice, Ψk(r) the waveIn Equation function (7), describing ħ represent the the given Dirac electron, constantEk andħ=hm⁄(its2π energy) , r is a and vector mass of respectively.the direct lattice, In the  case() theofa wave periodic function crystal, descri thebing Bloch’s the theoremgiven electron, [38–40 ]E givesk and the m formits energy of the and solutions mass respectively. of the Schrödinger In the caseequation. of a periodic There are crystal, domains the of Bloch’s energy Etheoremk where [38–40] no solution gives exists. the form These of domains the solutions are forbidden of the Schrödingerelectrons cannot equation. have energiesThere are with domains values of belonging energy Ek to where these regions.no solution Above exists. and These below domains this energy are forbiddengap are permitted electrons energy cannot regions have energies called bands. with valu In eaches belonging solid material to these there regions. are two Above important and below bands thisof energy energy gap are permitted energy regions called bands. In each solid material there are two importantValence bands Band of (VB): energy the range of permissible energy values that are the highest energies an electron can haveValence and Band(VB): still be associated the range with of a permissible particular atom ener ofgy a solidvalues material. that are the highest energies an electronConduction can have Band and still (CB): be the associated range of permissiblewith a particular energy atom values of whicha solid an material. electron in a solid material can haveConduction that allows Band the (CB): electron the torang dissociatee of permissible from a particular energy atomvalues and which become an aelectron free charge in a carrier solid materialin the material. can have that allows the electron to dissociate from a particular atom and become a free chargeThe carrier energy in the diff material.erence between the highest occupied energy state of the valence band and the lowestThe unoccupied energy difference state of thebetween conduction the highest band isoccupi calleded the energy band state gap E ofg isthe equal valence to: band and the lowest unoccupied state of the conduction band is called the band gap Eg is equal to: Eg = EVB ECB (8) Eg = EV B -− ECB (8)

In Equation (8) Eg is the bandgap energy, one the most important parameters in semiconductor In Equation (8) Eg is the bandgap energy, one the most important parameters in semiconductor physics. In metals one energy bands are partially filled; no bandgap energy between Valence and physics. In metals one energy bands are partially filled; no bandgap energy between Valence and conductive band is present. The energy level separating the occupied and unoccupied states is known conductive band is present. The energy level separating the occupied and unoccupied states is known asas the the Fermi Fermi level. level. Since Since there there are are available available states states immediately immediately above above the the Fermi Fermi surface, surface, it it requires requires an an infinitesimalinfinitesimal amount amount of of energy energy to to excite excite an an electron electron from from one one of of the the highest highest occupied occupied states states to to one one of of thethe lowest lowest unoccupied states.states. Depending onon thethe magnitudemagnitude ofof thethe band band gap gap energy, energy, di differentfferent types types of ofconductors conductors are are obtained. obtained. Metals, Metals, semiconductors semiconductors and and insulators insulators are are distinguished distinguished from from each each others others by bytheir their electronic electronic band band structures. structures. Their Their band band structures structures are are shown shown in thein the Figure Figure5. 5.

FigureFigure 5. 5. EnergyEnergy bands bands of of electrons electrons in in a a metal, metal, semicond semiconductoructor and and insulator. insulator. St Startingarting from from the the Fermi Fermi functionfunction [41], [41], the probabilityprobability ofof occupationoccupation of of energy energy levels levels in in valence valence band band and and conduction conduction band band is the is theFermi Fermi level. level. In In pure pure semiconductor semiconductor (named (named intrinsic), intrinsic), the the number number of ofholes holes inin valencevalence band is equal equal toto the the number number of of electrons electrons in in the conduction band band.. Hence, Hence, the the probability probability of of occupation occupation of of energy energy levelslevels in in conduction conduction band band and and va valencelence band band are are equal. Therefore, Therefore, the the Fermi Fermi level level for for the the intrinsic intrinsic semiconductorsemiconductor lies lies in in the the middle middle of of forbidden forbidden band. band.

If the band gap is sufficiently small external excitations, such as light, can enable the ”jump” of an electron from the valence band to the conduction band. The energy of this electron increases by a value equal to Eg. Therefore, the minimum energy of the photon (hν) to cause this jump must be at least equal to Eg (energy conservation ⇒ hν > Eg ). When an electron jumps the gap from the valence band, it creates a vacancy or hole in the valence band of the atom (i.e., it is devoid of an electron

Crystals 2019, 9, x FOR PEER REVIEW 7 of 17 Crystals 2019, 9, 531 7 of 17 normally there). A hole, as one might imagine, is a positive charge relatively speaking, since it is the absence of a negative charge that would normally be there to give a net charge of zero. The process resultsIf in the the band production gap is su ffiofciently an electron small hole external pair excitations,(see Figure such6a). as light, can enable the “jump” of an electronThe fromlight-emission the valence process band to in the a semiconductor conduction band. is quite The energy simple: of when this electron there is increases an electron by ain value the conductionequal to Eg .band Therefore, and an theempty minimum state in energy the valenc of thee band, photon the ( hconduction-banν) to cause thisd jump electron must can be relax at least to equal to Eg (energy conservation hν > Eg). When an electron jumps the gap from the valence band, fill the empty state in the valence band,⇒ the energy difference (i.e. the band gap Eg) being released as anit createsemitted a photon vacancy (see or hole Figure in the6b). valence In other band words, of the the atom electron (i.e., itand is devoidthe hole of recombine an electron to normally emit a there). A hole, as one might imagine, is a positive charge relatively speaking, since it is the absence of photon with energy approximately equal to the bandgap energy Eg: a negative charge that would normally be there to give a net charge of zero. The process results in the production of an electron hole pair (seeℎ∙=ℎ∙/ Figure6a).    ℎ ∙ / (9)

Figure 6. Simplified band diagram of a semiconductor. When a radiative transition occurs emitting Figure 6. Simplified band diagram of a semiconductor. When a radiative transition occurs emitting a a photon with an energy Eg h ν.(a) electron-hole pair production. (b) electron-hole pair recombination ≈⋅ · photonwith photon with an emission. energy Eg ≈ h ν. (a) electron-hole pair production. (b) electron-hole pair recombination with photon emission. The light-emission process in a semiconductor is quite simple: when there is an electron in the conductionIn order band to increase and an emptythe photon state influx the there valence must band, be many the conduction-band electrons in the electron conduction can relax band to (or fill holesthe empty in the statevalence in the band). valence In other band, words, the energy the semiconductor difference (i.e., must the band be "conductive"[42]. gap Eg) being released as an emittedThe photonsemiconductors (see Figure can6b). be In made other "more" words, conducti the electronve either and the by holeputting recombine extra electrons to emit a into photon the conductionwith energy band approximately or by removing equal electrons to the bandgap from th energye valenceEg :band; this can be manufactured through the doping. Doping is the process of introducing new energy levels into the band gap. Doping can be affected by substituting a foreign elementEg h ν into= h thec/ λsemiconductorλ h c/ Elattice.g Two types of doping can be(9) ≈ · · ⇔ ≈ · distinguished, n-type and p-type. In n-type doping, occupied donor levels are created very near the conductionIn order band to increase edge. Likewise, the photon p-type flux there doping must corresponds be many electrons to the in formation the conduction of empty band acceptor (or holes levelsin the near valence the band).valence In band other edge. words, The the acceptor semiconductor levels trap must electrons be “conductive” from the valence [42]. band, creating positiveThe charges semiconductors as carriers. can For be n- madetype “more”dopants, conductive the energy either band byis very putting close extra to the electrons bottom into of the the intrinsicconduction conduction band or band, by removing and room electrons temperature fromthe provides valence enough band; thisenergy can to be promote manufactured electrons through from thethe dopant doping. to Doping conduction is the band process and of improves introducing conduc newtivity energy (Figure levels 7) into by flowing the band through gap. Doping the partially can be filledaffected bands. by substituting The same is atrue foreign for p-type element dopant into thematerials semiconductor except that lattice. the dopant Two types energy of level doping is empty can be anddistinguished, very close to n-type the top and of p-type.the intrinsic In n-type valence doping, band occupiedallowing donorthe valence levels electrons are created to be very promoted near the toconduction the dopant band band edge. and Likewise, improve p-type conductivity. doping corresponds Un-doped to thesemiconductors formation of emptyare called acceptor intrinsic levels semiconductors.near the valence When band edge. a semiconductor The acceptor is levels dope trapd with electrons impurities, from it the becomes valence extrinsic. band, creating positive charges as carriers. For n-type dopants, the energy band is very close to the bottom of the intrinsic conduction band, and room temperature provides enough energy to promote electrons from the dopant to conduction band and improves conductivity (Figure7) by flowing through the partially filled bands. The same is true for p-type dopant materials except that the dopant energy level is empty and very close to the top of the intrinsic valence band allowing the valence electrons to be promoted to the

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Crystals 2019, 9, 531 8 of 17

dopant band and improve conductivity. Un-doped semiconductors are called intrinsic semiconductors. WhenCrystals a2019 semiconductor, 9, x FOR PEER REVIEW is doped with impurities, it becomes extrinsic. 8 of 17

Figure 7. (a) n-type semiconductors have dopant atoms with one “extra” electron mixed into the material. This forms a dopant band just below the conduction band allowing ionization of electrons to the conduction band with room temperature thermal energy. The extra electrons that are charge carriers in the circuit are represented by dots. The holes are also charge carriers in the circuit and are represented by circles. (b) p-type semiconductors have dopant atoms with one “less” electron mixed into the material. This forms an empty dopant band just above the valence band.

When p- and n-type semiconductors are brought into contact, a p-n junction (or diode) is formed. OnceFigure the p-n 7. junction((aa)) n-type n-type is semiconductors semiconductorsfirst formed, mobile have have dopant dopantcarriers atoms atoms (electron with with in one n-region “extra” electrone hole in mixed p-region) into thediffuse acrossmaterial. the junction This This forms forms (due a a todopant the concentrationband just below grad the conduction ients). Holes band diffuse allowing from ionization the p-region of electrons to the n- region,to the leaving conduction behind band negatively with room charged temperature immob th thermalermalile acceptor energy. Theions. extra Electrons electrons diffuse that are from charge the n- regioncarriers to the in p-region, the circuit leaving are represented behind positively by dots. The charged ho holesles are immobile also charge donor carriers ions. in the A region circuit anddepleted are of mobilerepresented carriers is by formed circles. ( bat)) p-type the junction. semiconductors The sp acehave charge dopant due atoms to with immobile one “less” ions electron in the mixeddepletion regioninto establishes the material. a diffusion This This forms forms pote an an empty emptyntial is do dopant generatedpant band (seejust aboveFigure the 8). valence band. Excess negative charge forms in the p-region of the junction, and excess positive charge forms When p- and n-type semiconductors are brought into contact, a p-n junction (or diode) is formed. in theWhen n-region p- and of n-typethe junction, semiconductors a barrier that are broughteffectively into prevents contact, morea p-n junctioncarriers from(or diode) moving is formed. across Once the p-n junction is first formed, mobile carriers (electron in n-region e hole in p-region) diffuse theOnce junction. the p-n junctionThe diffusion is first potential formed, mobileprevents carriers more (electroncarriers fromin n-region moving e hole across in p-region)the junction. diffuse In across the junction (due to the concentration gradients). Holes diffuse from the p-region to the n-region, chemicalacross the equilibrium junction (due (no toexternal the concentration bias) the Fermi’s grad leveients).l, is Holes a constant diffuse across from the the junction. p-region In toturn, the the n- leaving behind negatively charged immobile acceptor ions. Electrons diffuse from the n-region to the energiesregion, leaving of the conduction behind negatively and valence charged bands immob for eachile ofacceptor the materials ions. Electrons(p and n type) diffuse will from be moved, the n- p-region, leaving behind positively charged immobile donor ions. A region depleted of mobile carriers withregion the to levelsthe p-region, of the n-typeleaving material behind positively falling. A charged contact immobilepotential donor(Vj) develops, ions. A regionbetween depleted the two of is formed at the junction. The space charge due to immobile ions in the depletion region establishes materials,mobile carriers a voltage is formed that is requiredat the junction. to be placed The sp acrossace charge the junction due to to immobile make electrons ions in jump the depletion the band a diffusion potential is generated (see Figure8). gapregion and establishes allow current a diffusion to flow. pote Thisntial is the is generatedbias voltage (see of Figurethe diode. 8). Excess negative charge forms in the p-region of the junction, and excess positive charge forms in the n-region of the junction, a barrier that effectively prevents more carriers from moving across the junction. The diffusion potential prevents more carriers from moving across the junction. In chemical equilibrium (no external bias) the Fermi’s level, is a constant across the junction. In turn, the energies of the conduction and valence bands for each of the materials (p and n type) will be moved, with the levels of the n-type material falling. A contact potential (Vj) develops, between the two materials, a voltage that is required to be placed across the junction to make electrons jump the band gap and allow current to flow. This is the bias voltage of the diode.

FigureFigure 8. ((aa)) n-type n-type semiconductors semiconductors have have dopant dopant atoms atoms with with one one “extra” “extra” electron electron mixed mixed into into the material.material. This This forms forms a a dopant dopant band band just just below below the the conduction conduction band band allowing allowing ionization ionization of of electrons to the conduction band with room temperature thermal energy. The extra electrons that are charge to the conduction band with room temperature thermal energy. The extra electrons that are charge carriers in the circuit are represented by dots. (b) At equilibrium the two current (drift and diffusion) carriers in the circuit are represented by dots. (b) At equilibrium the two current (drift and diffusion) have the same magnitude, but flow in opposite directions. have the same magnitude, but flow in opposite directions. Excess negative charge forms in the p-region of the junction, and excess positive charge forms in the n-region of the junction, a barrier that effectively prevents more carriers from moving across the junction. The diffusion potential prevents more carriers from moving across the junction. In chemical Figure 8. (a) n-type semiconductors have dopant atoms with one “extra” electron mixed into the equilibrium (no external bias) the Fermi’s level, is a constant across the junction. In turn, the energies material. This forms a dopant band just below the conduction band allowing ionization of electrons of the conduction and valence bands for each of the materials (p and n type) will be moved, with the to the conduction band with room temperature thermal energy. The extra electrons that are charge carriers in the circuit are represented by dots. (b) At equilibrium the two current (drift and diffusion) have the same magnitude, but flow in opposite directions.

Crystals 2019, 9, 531 9 of 17

levels of the n-type material falling. A contact potential (V ) develops, between the two materials, Crystals 2019, 9, x FOR PEER REVIEW j 9 of 17 a voltage that is required to be placed across the junction to make electrons jump the band gap and allowWhen current the todiode flow. is This forward is the biasbiased, voltage meaning of the that diode. a potential large enough to overcome the contactWhen potential the diodeof the junction is forward is applied, biased, electrons meaning are that supplied a potential to the large n-type enough material to overcomeand holes to the thecontact p-type potential material of (i.e., the junctioncurrent flow is applied, through electrons the p-n are junction). supplied To to maintain the n-type electrical material equilibrium, and holes to thethe excess p-type injected material carriers (i.e., currentare removed flow throughby recombinat the p-nion junction). of electron To hole maintain pairs electricalin which electrons equilibrium, in thethe conduction excess injected band carriers fall to the are valence removed band, by recombination the result being of the electron emission hole of pairs a photon in which (in the electrons case of in anthe LED) conduction or the bandproduction fall to the of valenceheat by band, a non-radi the resultative being process the emission (in most of adiodes photon used (in the for case signal of an applications).LED) or the production Figure 9 shows of heat the by flow a non-radiative of electrons process and holes (in most across diodes the junction used for in signal a biased applications). diode. ThereFigure are9 shows many varieties the flow ofof electronsextrinsic semiconductor and holes across materials the junction available in a biased for LEDs diode. to create There a are specific many colorvarieties of light. of extrinsic semiconductor materials available for LEDs to create a specific color of light.

Figure 9. LED p-n junction at forward bias. Figure 9. LED p-n junction at forward bias. The main semiconductor materials used to manufacture LEDs are [43]: The main semiconductor materials used to manufacture LEDs are [43]:  .Aluminum Aluminum gallium gallium arsenide arsenide (AlGaAs): (AlGaAs): infrared infrared and red;and red;  .Gallium Gallium phosphide phosphide (GaP): (GaP): yellow yellow and green;and green;  .Aluminum Aluminum gallium gallium indium indium phosphide phosphide (AlGaInP): (AlGaInP): red, orangered, orange and yellow;and yellow;  .Indium Indium gallium gallium nitride nitride (InGaN): (InGaN): green, green, blue andblue ultraviolet.and ultraviolet.

3.2.3.2. LEDs LEDs as as Light Light Sensors Sensors InIn addition addition to toemitting emitting light, light, an LED an LED can canbe us beed usedas a photodiode as a photodiode light sensor/detector. light sensor/detector. This capabilityThis capability may be may used be usedin a invariety a variety of applications of applications including including ambient ambient light light level level sensor sensor and and bidirectionalbidirectional communications. communications. As aa photodiode,photodiode, an an LED LED is sensitiveis sensitive to wavelengths to wavelengths equal equal to or shorterto or shorterthan the than predominant the predominant wavelength wavelength it emits. it emits. WhenWhen photons photons with with energy energy greater greater than than bandgap bandgap fa fallll on on the the depletion depletion region region of of LED, LED, they they are are absorbed,absorbed, and and electron-hole electron-hole pairs pairs are are created created (see (see Figu Figurere 10a). 10a). The The electric electric field field across across the the region region pulls pulls themthem apart apart causing causing a tiny a tiny current current to flow. to flow. The The more more photons photons (light) (light) the more the more the current the current adds up adds and up soand you so have you a have current a current that is proportional that is proportional to the ligh to thet falling light on falling the diode, on the this diode, is called this a is photo called current a photo (seecurrent Figures (see 10b Figure and 1010c).b,c). As As already already mentioned, mentioned, th thee phenomenon phenomenon occurs occurs only only if ifthe the energy energy of of the the incidentincident photons photons is is greater greater than than the the energy energy gap gap of of the the semiconductor semiconductor with with which which the the LED LED is is made made (internal photoelectric effect). Therefore, using light with different wavelengths it is possible to verify this important effect.

Crystals 2019, 9, 531 10 of 17 Crystals 2019, 9, x FOR PEER REVIEW 10 of 17 Crystals 2019, 9, x FOR PEER REVIEW 10 of 17 (internal photoelectric effect). Therefore, using light with different wavelengths it is possible to verify this important effect.

FigureFigure 10. 10.LED LED p-n p-n junction junction used used as as light light sensor. sensor. (a ()a creation) creation of of electron-hole electron-hole pairs pairs (b )(b phenomenon) phenomenon of theFigure photo-current 10. LEDof p-n the (c) junction sketchphoto-current of used a LED as (c) inlight sketch which sensor. of a a photo-current LED(a) creation in which of flows. aelectron-hole photo-current pairs flows. (b) phenomenon of the photo-current (c) sketch of a LED in which a photo-current flows. 4. Apparatus 4. Apparatus 4. ApparatusIn this work, we have implemented a simple tool, based on an LED-LED structure, suitable for In this work, we have implemented a simple tool, based on an LED-LED structure, suitable for demonstrating the internal photoelectric effect in a simple and economical way. demonstratingIn this work, the we internal have photoelectricimplemented effecta simple in a tool, simple based and on economical an LED-LED way. structure, suitable for A diagram showing in schematic the various components of the implement apparatus is in demonstratingA diagram the showing internal in photoelectric schematic the effect various in a simple components and economical of the implement way. apparatus is in Figure 11. The system is realized with two distinct functional sections: (a) piloting of the LED used as FigureA 11.diagram The system showing is realized in schematic with two the distinctvarious functional components sections: of the (a) implement piloting of apparatus the LED usedis in a light transmitter; (b) LEDs used as light receivers. asFigure a light 11. transmitter; The system (b) is LErealizedDs used with as twolight distinct receivers. functional sections: (a) piloting of the LED used as a light transmitter; (b) LEDs used as light receivers.

Figure 11. Schematic representation of realized system demonstration of the photoelectric effect. Figure 11. Schematic representation of realized system demonstration of the photoelectric effect. The The system is realized with two distinct functional sections: (a) piloting of the LED used as a light systemFigure 11.is realizedSchematic with representation two distinct of functionalrealized system sections: demonstration (a) piloting of of the the photoelectric LED used effect.as a light The transmitter; (b) LEDs used as light receivers. transmitter;system is realized (b) LEDs with used two as lightdistinct receivers. functional sections: (a) piloting of the LED used as a light transmitter; (b) LEDs used as light receivers.

Crystals 2019, 9, 531 11 of 17 CrystalsCrystals 2019 2019, ,9 9, ,x x FOR FOR PEER PEER REVIEW REVIEW 1111 of of 17 17

The realized apparatus is shown in Figure 12. The system is designed to be able to use different TheThe realizedrealized apparatus apparatus isis shownshown ininFigure Figure 12 12.. TheThe systemsystem isis designeddesigned toto bebe ableable toto useuse didifferentfferent LEDs as light emitters. Two insulated sockets allow you to connect the LED of interest to the LEDsLEDs as as light light emitters. emitters. Two Two insulated insulated sockets sockets allow allo youw to you connect to connect the LED ofthe interest LED of to theinterest instrument. to the instrument.instrument.

Figure 12. (a) The apparatus used for demonstration of the photoelectric effect. (b) Section used to FigureFigure 12.12. ((aa)) TheThe apparatusapparatus usedused forfor demonstrationdemonstration ofof thethe photoelectricphotoelectric effect.effect. ((bb)) SectionSection usedused toto control the LED used as light source. (c) Section used to control the LEDs used as receivers. controlcontrol the the LED LED used used as as light light source. source. ( (cc)) Section Section used used to to control control the the LEDs LEDs used used as as receivers. receivers. In the realized system, the section used to control the LED used as light transmitter is being In the realized system, the section used to control the LED used as light transmitter is being implementedIn the realized according system, the circuit the section shown used in the to Figure control 13 .the LED used as light transmitter is being implementedimplemented accordingaccording thethe circuitcircuit shownshown inin thethe FigureFigure 13.13.

Figure 13. Circuit used for driving the LEDs used as light source (Tx section). FigureFigure 13. 13. Circuit Circuit used used for for driving driving the the LEDs LEDs used used as as light light source source (Tx (Tx section). section). The circuit shown in Figure 13 drivers the LED Tx, used as a light source, with current supply. TheThe circuitcircuit shownshown inin FigureFigure 1313 driversdrivers thethe LEDLED TxTx,, usedused asas aa lightlight source,source, withwith currentcurrent supply.supply. The key to the circuit’s operation is in placing a current sensing resistor (in our circuit RSENSE = 270 ΩΩ) TheThe keykey toto thethe circuit'scircuit's operationoperation isis inin placingplacing aa currentcurrent sensingsensing resistorresistor (in(in ourour circuitcircuit RRSENSESENSE == 270270 Ω)) in the op amp (OP 27) feedback loop. The current delivered to the LED Tx is inin thethe opop ampamp (OP(OP 27)27) feedbackfeedback loop.loop. TheThe currentcurrent delivereddelivered toto thethe LEDLED TxTx isis

ILED = Vin/RSENSE (10) IILEDLED =V =Vinin⁄⁄RRSENSESENSE (10)(10)

TheThe used usedused circuit circuitcircuit works worksworks on onon two twotwo simple simplesimple principles principlesprinciples (we (we remember(we rememberremember that thethatthat voltages thethe voltagesvoltages at the twoatat thethe inputs twotwo areinputsinputs the aresameare thethe and samesame no and currentand nono current flowscurrent into flowsflows the intointo op amp’s thethe opop input amp'samp's terminals inputinput terminalsterminals [44]). [44]).[44]). 1.1. The The opop ampamp adjustsadjusts itsits outputoutput toto bringbring itsits negativenegative inputinput equalequal toto thethe positivepositive input.input. ThisThis 1.meansmeansThe thatthat op tensiontension amp adjusts acrossacross its thethe output RRSENSESENSE to itit bring isis equalequal its atnegativeat thethe inputinput input voltagevoltage equal VV toinin.. the positive input. This means 2.that2. tensionA A simplesimple across applicationapplication the RSENSE ofof OhmsOhmsit is equal lawlaw nownow at the tellstells input youyou voltage thethe currentcurrentVin. inin RRSENSESENSE:: 2. A simple application of Ohms law now= tells⁄ you⁄ the current= in RSENSE: = = (11)(11)

SENSE thethe samesame currentcurrent thatthat isis developeddevelopedIR throughSENSEthrough= V RRinSENSE/RSENSE mustmust= alsoalsoILED flowflow throughthrough thethe LED.LED. (11)

Crystals 2019, 9, 531 12 of 17

Crystals 2019, 9, x FOR PEER REVIEW 12 of 17 the same current that is developed through RSENSE must also flow through the LED. TheThe emittedemitted lightlight ofof thethe LEDLED isis directlydirectly proportionalproportional to the current that passes through them. ForFor this reason, the the possibility possibility to to modify modify the the intensit intensityy of ofthe the emitted emitted light light is a isvery a very important important test case test caseallows allows showing showing that the that "birth" the “birth” of the photoelectric of the photoelectric effect does effect not does depend not dependon the luminous on the luminous intensity intensitybut starts but to startsappear to appearonly if onlythe iflength the length of the of ra thediation radiation is the is the appropriate appropriate one. one. This This allows allows us toto demonstratedemonstrate thatthat thethe energyenergy acquiredacquired byby thethe singlesingle electronelectron isis independentindependent ofof thethe amplitudeamplitude ofof thethe lightlight wave.wave. WhenWhen the the intensity intensity of of the the light light increases, increases, if the if the energy energy of the of incidentthe incident photons photons is larger is larger than thethan forbidden the forbidden band, theband, number the ofnumber electrons of thatelectr superons that the energysuper gapthe increasesenergy gap (the increases photocurrent (the increases).photocurrent If the increases). energy of If thethe incidentenergy of photon the incident is less photon than the is energy less than gap the presents energy from gap presents valence bandfrom tovalence conduction band band,to conduction whatever band, the intensity whatever of thethe radiation intensity emitted of the radiation by the LED, emitted the photoelectric by the LED, eff theect doesphotoelectric not occur effect (the photocurrentdoes not occur is (the zero). photocurrent is zero). TheThe realizedrealized systemsystem allowsallows modifying,modifying, throughthrough multi-turnmulti-turn trimmer,trimmer, toto “linearly”"linearly" thethe currentcurrent intensityintensity flowingflowing inin thethe LEDLED (light(light intensityintensity emittedemitted proportionalproportional toto current).current). Furthermore,Furthermore, aa switchswitch allowsallows switchingswitching thethe inputinput fromfrom thethe continuouscontinuous sourcesource toto anan externalexternal waveformwaveform/function/function generator. generator. TheThe sectionsection usedused toto control control the the LED LED used used as as light light receiver receiver is is achieved achieved according according circuit circuit shown shown in thein the Figure Figure 14. 14.

Figure 14. Circuit used to convert light into output voltage. Figure 14. Circuit used to convert light into output voltage. In this circuit LEDs are used as light sensors (Rx section). TheIn this Rx circuit section, LEDs used are to convertused as thelight incident sensors light (Rx intosection). an electrical signal, is made with five LEDs of diffTheerent Rx colors section, (the used LEDs to convert are used the as incident wavelength-selective light into an electrical light sensors). signal, is made with five LEDs of differentBy means colors of the (the Tx LEDs section, are light used with as wavelength-selective different wavelength light (using sensors). LEDs with different colors) can be produced.By means This of the light Tx can section, hit the light LEDs with of thedifferent Rx section. wavelength If the wavelength(using LEDs of with the incidentdifferent lightcolors) is greatercan be produced. than or equal This to light that can which hit the the LEDs LEDs of used the Rx as lightsection. sensors If thecould wavelength emit, in of the thelatter incident a photo light currentis greater is produced.than or equal Coupling to that which LEDs bythe varying LEDs used the color,as light it issensors possible could to evaluate emit, in thethe generationlatter a photo of ancurrent electric is produced. current or not.Coupling To detect LEDs the by emitted varying photocurrent the color, it is of possible LEDs, a to low evaluate noise OP the 27 generation operational of amplifieran electric is current used in or configuration not. To detect current the emitted to voltage photocurrent converter, of i.e., LEDs, a transimpedance a low noise OP amplifier. 27 operational In the proposedamplifier is circuit, used in the configuration output voltage current of operational to voltage converter, amplifier (i.e.VOUT a transimpedance) is directly proportional amplifier. toIn thethe current given to the inverting terminal of the op amp (the photocurrent generated by enlightened LED). proposed circuit, the output voltage of operational amplifier (VOUT) is directly proportional to the currentResistors given haveto the been inverting inserted terminal in series of with the the op LEDs. amp (the From photocurrent the point of view generated of the transimpedanceby enlightened amplifierLED). they are inessential. On the other hand, they are fundamental for being able to “turn on” Resistors have been inserted in series with the LEDs. From the point of view of the transimpedance amplifier they are inessential. On the other hand, they are fundamental for being able to “turn on” the LEDs to be able to control the light’s color emitted. Indeed, before beginning the

Crystals 2019, 9, 531 13 of 17 the LEDs to be able to control the light’s color emitted. Indeed, before beginning the educational experiments, the switch named LED Test can be used to check the color of the light emitted by the LEDs. All to check that what will then be studied is consistent with what was expected. In our system, a self-made structure is positioned above the Rx LEDs in order to facility the coupling between Rx and Tx LED. Considering the properties of the operational amplifier, when an LED is hit by appropriate radiation, it generates photocurrent; the wavelength of the incident radiation must be less than the wavelength of the light that the LED can emit. Due to the properties of the operational amplifier, this photocurrent flows in the feedback resistance (in our circuit Rf = 1 MΩ). For this reason, the output voltage of the operational amplifier is: Vout = I R (12) ph· f In Equation (12), Iph represents the generated photocurrent, Rf is the feedback resistor and VOUT is the op amp output voltage. By means of circuit illustrated in Figure 14, we have a voltage output proportional to the light intensity of the Tx LED. Our system allows two important checks to be carried out on the internal photoelectric effect.

 Verify that the photoelectric effect occurs only if hν > Eg.  Prove that the energy carried by the light is quantized.

The tests were carried out by lighting a single receiving LED with a single transmitting LED. The two LEDs were positioned face-to-face by means of a self-made structure. When a wavelength is detected, a photocurrent proportional to the incident power is generated; the photocurrent is proportional to the responsivity of the led. In particular, it is possible to verify that the photocurrent is generated only if the energy of the incident photons is greater than the bandgap energy of the semiconductor used to realize the receiving LED. In other words, if the wavelength of the LED illuminator is less than the wavelength that would “emit” LED receiver. For example, with our system it is possible to verify that if the yellow LED is illuminated with red light there is no generation of photocurrent, on the contrary it generates photocurrent if it irradiates with yellow, green, blue or UV radiation. Another important test that can be carried out is to verify that we have hνTx < Eg Rx, whatever the − luminous intensity of the incident radiation. For example, if we use Red LED to light up a yellow LED, the photocurrent is not generated, any intensity of red light is used. The system is able to check that the generated current is proportional to the number of incident photons. Using the multi-turn trimmer, we can vary the current flowing in the transmitter LED. The luminous intensity (the number of photons emitted) is proportional to the bias current. If we couple the transmitter LED with the same type of LED used as a receiver, we can verify that the generated photocurrent is proportional to the incident luminous intensity. List of LED used as receivers and transmitters is shown in Table1.

Table 1. List of LEDs used in the systems.

Color Model Dominant Wavelength (nm) Technology Red Kingbright_ L-7113SEC-E 630 nm AlGaInP Yellow Kingbright_L-53SYC 595 nm AlGaInP Green Nichia NSPG520AS 520 nm InGaN Blue Kingbright_L-7113QBC-D 465 nm InGaN Violet BIVAR UV5TZ-390-30 390 nm InGaN

Figure 15 shows the emission spectra of the used LEDs. Crystals 2019, 9, 531 14 of 17 Crystals 2019, 9, x FOR PEER REVIEW 14 of 17

Figure 15. Emission spectra of the used LEDs. Figure 15. Emission spectra of the used LEDs. 5. LED-to-LED Communication 5. LED-to-LED Communication Current studies on the possibility of using Visible Light Communication (VLC) has received great attentionCurrent from studies industry on andthe possibility the academic of using community Visible [ 45Light–49]. Communication VLC systems transmit (VLC) has information received wirelesslygreat attention by rapidly from pulsing industry visible and light the usingacademic light emittingcommunity diodes [45–49]. (LEDs). VLC systems transmit informationSome recent wirelessly works by showed rapidly thepulsing possibility visible tolight make using a connectionlight emitting by diodes the Light (LEDs). Fidelity (Li-Fi) technology.Some recent Li-Fi is works a “post-Wi-Fi” showed wirelessthe possibility technology to ma basedke a on connection the use of Visibleby the LightLight Communication Fidelity (Li-Fi) (VLC)technology. (instead Li-Fi of is radio a “post-Wi-Fi” frequency waves wireless for technology Wi-Fi). Li-Fi based is aon VLC the system use of that Visible uses Light light fromCommunication Light-Emitting (VLC) Diodes (instead (LEDs) of radio as frequency a medium wa toves deliver for Wi-Fi). networked, Li-Fi is mobile a VLC system and high-speed that uses communications.light from Light-Emitting Li-Fi principle Diodes relies(LEDs) on as the a medium data transmission to deliver networked, by amplitude mobile modulation and high-speed of light sourcescommunications. [50–52]. The Li-Fi Li-Fi prin technologyciple relies can on be the used data for varioustransmission purposes, by amplitude it matters the modulation data transmission of light throughsources [50–52]. LEDs thus The all Li-Fi the screenstechnology which can illuminate be used lightfor various can be purposes, served as ait platformmatters the for datadata communication.transmission through The LEDs screen thus of all the the mobile screens phone, which television,illuminate light bulbs can can be act served as a as source a platform of light. for Ondata the communication. other hand, the The receiving screen platform,of the mobile the photo phone, detector television, can bebulbs replaced can act by as a cameraa source in of mobile light. phoneOn the forother scanning hand, the and receiving retrieving platform, data. the This photo technology detector hascan thebe replaced advantage by a of camera being in useful mobile in electromagneticphone for scanning sensitive and areasretrieving such asdata. in aircraft This technology cabins, in hospitals,has the advantage nuclear sites, of withoutbeing useful causing in electromagnetic interference.sensitive areas such as in aircraft cabins, in hospitals, nuclear sites, without causing electromagneticIn proposed interference. system, through an external function generator, it is possible to modulate the powerIn supplyproposed of thesystem, LED through (see the an circuit external shown function in Figure generator, 13). In it order is possible to demonstrate to modulate the the ability power to transmitsupply of/receive the LED information (see the via circuit LEDs, shown the Tx in LED Figure current 13). has In been order modulated to demonstrate with a square the ability wave atto 1transmit/receive MHz. Such modulated information light via beam LEDs, was the received Tx LED by current the light has receiving been modulate LED. Thed Figurewith a 16square shows wave the waveformsat 1 MHz. Such of the modulated source signal light (Tx—red beam was waveform) received andby the the light output receiving voltage LED. obtained The fromFigure the 16 LED shows Rx (bluethe waveforms waveform). of the source signal (Tx - red waveform) and the output voltage obtained from the LED Rx (blue waveform).

Crystals 9 Crystals2019, 2019, 531, 9, x FOR PEER REVIEW 15 of 1715 of 17

Figure 16. In yellow, waveform used to modulate the light intensity of the LED used as Tx. In red, Figure 16. In yellow, waveform used to modulate the light intensity of the LED used as Tx. In red, waveform of the signal detected by the LED used as Rx. Yellow LED was employed as light source and waveform of the signal detected by the LED used as Rx. Yellow LED was employed as light source red LED was used as receiver. and red LED was used as receiver. The LEDs have very low parasitic capacities. This property makes them suitable to realize optical communicationsThe LEDs with have high very bit low rate. parasitic Data transmission capacities. This at rates property exceeding makes 100 them Mbit suitable/s is possible to realize using optical LEDscommunications as transmitter and with receiver high bit [53 rate.]. Data transmission at rates exceeding 100 Mbit/s is possible using LEDs as transmitter and receiver [53]. 6. Conclusions 6. Conclusions A low-cost device for experimental demonstration of photoelectric effect has been proposed. Tool basedA both low-cost on the device light emission for experimental/detection capabilitydemonstratio of then of LEDs. photoelectric The simplicity effect and has safetybeen ofproposed. the deviceTool allows based it toboth be on widely the light used emission/detection for the didactic demonstration capability of ofthe the LEDs. corpuscular The simplicity nature and of light. safety of Furthermore,the device the allows tool allows it to be to widely familiarize used with for the the di employmentdactic demonstration of LEDs as of selective the corpuscular light detector. nature of This propertylight. Furthermore, of LEDs is extremelythe tool allows interesting to familiarize to realize thewith optical the employment wireless communication of LEDs as (Li-Fi);selective an light interestingdetector. alternative This property to the of classic LEDs Radio is extremely Frequency intere wirelesssting to Communication realize the optical (Wi-Fi). wireless communication (Li-Fi); an interesting alternative to the classic Radio Frequency wireless Communication (Wi-Fi). Author Contributions: Conceptualization, G.S.S.; methodology, G.S.S.; validation, G.S.S. and F.L.; formal analysis, F.L.; investigation,Author Contributions: G.S.S. and For M.L.; research resources, articles F.L.; with data curation,several authors, M.L.; writing—original a short paragraph draft specif preparation,ying their G.S.S.individual and M.L.;contributions writing—review must be and provided. editing, F.L. The and following G.S.S.; visualization, statements F.L.;should supervision, be used F.L.; “conceptualization, funding acquisition, G.S.S.; F.L. methodology, G.S.S.; validation, G.S.S. and F.L.; formal analysis, F.L.; investigation, G.S.S. and M.L.; resources, Funding:F.L.; Thisdata researchcuration, received M.L.; writing—orig no externalinal funding draft and preparation,G.S.S. The APC was funded and M.L. by; Fabiowriting—review Leccese. and editing, F.L. and G.S.S.; visualization, F.L..; supervision, F.L..; funding acquisition, F.L..” Conflicts of Interest: The authors declare no conflict of interest. Funding: Please add: “This research received no external funding” and “The APC was funded by Fabio Leccese”.

ReferencesConflicts of Interest: “The authors declare no conflict of interest.” 1. Müller, R.; Wiesner, H. Teaching quantum mechanics on an introductory level. Am. J. Phys. 2002, 70, 200–209. 2. ReferencesPlaninši, G.; Etkina, E. Light-emitting diodes: A hidden treasure. Phys. Teach. 2014, 52, 94–99. [CrossRef] 3. 1.Schirripa Müller, Spagnolo, R.; Wiesner, G.; Martocchia, H. Teaching A.; quantu Papalillo,m mechanics D.; Cozzella, on L. an Simple introductory educational level, tool Am. for J. digitalPhys. 2002 speckle, 70, 200– shearography.209. Eur. J. Phys. 2012, 33, 733–750. [CrossRef] 4. 2.Hadzidaki, Planinši, P.; Kalkanis,G; Etkina, G.; E. Stavrou,Light-emitting D. Quantum diodes: mechanics: a hidden treasure, A systemic Phys. component Teach. 2014 of, the52, modern94–99. physics 3.paradigm. SchirripaPhys. Spagnolo, Educ. 2000 G.;, Martocchia,35, 386–392. A.; [CrossRef Papalillo,] D.; Cozzella, L. Simple educational tool for digital speckle 5. Feynman,shearography. R. QED: The Eur. Strange J. Phys. Theory 2012 of Light, 33, 733–750. and Matter ; Princeton University Press: Princeton, NJ, USA, 1985; p. 15. 6. 4.Klassen, Hadzidaki, S. The Photoelectric P.; Kalkanis, effect: G.; ReconstructingStavrou, D. Quantum the story mechanics: for the physics a systemic classroom. componentSci. Educ. of2011 the, 20modern, 719–731. physics [CrossRefparadigm.] Phys. Educ. 2000, 35, 386-392. 7. 5.Hertz, Feynman, H. Ueber R. strahlen QED: The electrischer Strange Theory kraft. Annalenof Light and der PhysikMatter1889; Princeton, 272, 769–783. University [CrossRef Press: ]Princeton, NJ, USA, 1985; p.15.

Crystals 2019, 9, 531 16 of 17

8. Lenard, P. Über die lichtelektrische Wirkung. Annalen der Physik 1902, 8, 149–198. [CrossRef] 9. Einstein, A. Einstein’s proposal of the photon concept—A translation of the annalen der physik paper of 1905. Am. J. Phys. 1965, 33, 1–16. 10. Boys, D.W.; Cox, M.E.; Mykolajenko, W. Photoelectric effect revisited (or an inexpensive device to determine h/e). Am. J. Phys. 1978, 46, 133. [CrossRef] 11. Garver, W.P. The photoelectric effect using LEDs as light sources. Phys. Teach. 2006, 44, 272–275. [CrossRef] 12. Loparco, F.; Malagoli, M.S.; Rainò, S.; Spinelli, P. Measurement of the ratio h/e with a photomultiplier tube and a set of LEDs. Eur. J. Phys. 2017, 38, 025208. [CrossRef] 13. Pimputkar, S.; Speck, J.S.; DenBaars, S.P.; Nakamura, S. Prospects for LED lighting. Nat. Photonics 2009, 3, 180–182. [CrossRef] 14. Khan, T.Q.; Bodrogi, P.; Vinh, Q.T.; Winkler, H. LED Lighting: Technology and Perception; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2015. 15. Papalillo, D.; Del Vecchio,P.; Schirripa Spagnolo, G. LED applications in road and railway signals: Is it possible to fit specifications? In Proceedings of the Photonics Prague 2011, Prague, Czech Republic, 11 October 2011. [CrossRef] 16. Schirripa Spagnolo, G.; Papalillo, D.; Martocchia, A.; Makary, G. Application of LEDs to traffic signal. In Proceedings of the 2012 11th International Conference on Environment and Electrical Engineering, Venice, Italy, 18–25 May 2012; pp. 864–868. [CrossRef] 17. Schirripa Spagnolo, G.; Papalillo, D.; Martocchia, A. Light emitting diode in stationary transportation applications: Wavelength response to varying temperature. In Proceedings of the SPIE OPTO 2012, San Francisco, CA, USA, 2 February 2012. 18. Schirripa Spagnolo, G.; Papalillo, D.; Malta, C.; Vinzani, S. LED railway signal vs full compliance with colorimetric specification. Int. J. Transp. Dev. Integr. 2017, 1, 568–577. [CrossRef] 19. Heber, J. Nobel Prize 2014: Akasaki, Amano & Nakamura. Nat. Phys. 2014, 10, 791. 20. Akasaki, I.; Amano, H.; Nakamura, S. Blue LEDs—Filling the World with New Light. R. Swed. Acad. Sci. Nobel Prize Phys. 2014. Available online: http://www.theo-physik.uni-kiel.de/~{}bonitz/D/vorles_14ws/ popular-physicsprize2014.pdf (accessed on 12 October 2019). 21. Mims, F.M. Sun photometer with light-emitting diodes as spectrally selective detectors. Appl. Opt. 1992, 31, 6965–6967. [CrossRef] [PubMed] 22. RayChaudhuri, B.; Sen, C. Light emitting diode as sensor for miniature multispectral radiometer. Appl. Phys. B 2009, 95, 141–144. [CrossRef] 23. Rajagopal, S.; Roberts, R.D.; Lim, S.K. IEEE 802.15.7 visible light communication: Modulation schemes and dimming support. IEEE Commun. Mag. 2012, 50, 72–82. [CrossRef] 24. Medina, C.; Zambrano, M.; Navarro, K. LED based visible light communication: Technology, applications and challenges-a survey. Int. J. Adv. Eng. Technol. 2015, 8, 482–495. 25. Incipini, L.; Belli, A.; Palma, L.; Ballicchia, M.; Piereoni, P. Sensing Light with LEDs: Performance Evaluation for IoT Applications. J. Imaging 2017, 3, 50. [CrossRef] 26. Matheus, L.E.M.; Vieira, A.B.; Vieira, L.F.; Vieira, M.A.; Gnawali, O. Visible light communication: Concepts, applications and challenges. IEEE Commun. Surv. Tutor. 2019, 1, 1. [CrossRef] 27. Whitefield, R.J.; Brady, J.J. New value for work function of sodium and the observation of surface- plasmon effects. Phys. Rev. Lett. 1971, 26, 380–383. [CrossRef] 28. Lloyd, D.R. What was measured in Millikan’s study of the photoelectric effect? Am. J. Phys. 2015, 83, 765–772. [CrossRef] 29. Sze, S.M.; Ming-Kwei Lee, M.K. Semiconductor Devices: Physics and Technology, 3rd ed.; John Wiley & Sons: Hoboken, NJ, USA, 2006. 30. Diaz, L.; Smith, C.A. Investigating the photoelectric effect using LEDs and a modular spectroscope. J. Chem. Educ. 2005, 82, 906–908. [CrossRef] 31. Miyazaki, E.; Itami, S.; Araki, T. Using a light-emitting diode as a high-speed, wavelength selective photodetector. Rev. Sci. Instrum. 1998, 69, 3751–3754. [CrossRef] 32. Rossiter, J.; Mukai, T. A novel tactile sensor using a matrix of LEDs operating in both photoemitter and photodetector modes. In Proceedings of the SENSORS, 2005 IEEE, Irvine, CA, USA, 31 October–3 November 2005. 33. O’Toole, M.; Diamond, D. Absorbance based light emitting diode optical sensors and sensing devices. Sensors 2008, 8, 2453–2479. [CrossRef] Crystals 2019, 9, 531 17 of 17

34. Jo, Y.C.; Kim, H.N.; Kang, J.H.; Hong, H.K.; Choi, Y.S.; Jung, S.W.; Kim, S.P. Novel wearable-type biometric devices based on skin tissue optics with multispectral LED–photodiode matrix. Jpn. J. Appl. Phys. 2017, 56. [CrossRef] 35. Petit, M.; Michez, L.; Raimundo, J.M.; Dumas, P. Electrical and optical measurements of the bandgap energy of a light-emitting diode. Phys. Educ. 2016, 51, 025003. [CrossRef] 36. Einstein, A. Relativity, 1st ed.; Routledge: London, UK, 2013. 37. Bloch, F.Z. Über die quantenmechanik der elektronen in kristallgittern. Zeitschrift für Physik 1929, 52, 555–600. [CrossRef] 38. Kronig, R.d.L.; Penney, W.G. Quantum mechanics of electrons in crystal lattices. Proc. R. Soc. Lond. A 1931, 130, 499–513. [CrossRef] 39. Herman, F. Theoretical investigation of the electronic energy band structure of solids. Rev. Mod. Phys. 1958, 30, 102–122. [CrossRef] 40. Shockley, W.; Last, J.T. Statistics of the charge distribution for a localized flaw in a semiconductor. Phys. Rev. 1957, 107, 392–396. [CrossRef] 41. Wagner, E.P. Investigating bandgap energies, materials, and design of light-emitting diodes. J. Chem. Educ. 2016, 93, 1289–1298. [CrossRef] 42. Lorenz, M.R. Visible light from semiconductors: Luminescence from p-n junctions and potential uses of solid state light sources are discussed. Science 1968, 159, 1419–1423. [CrossRef][PubMed] 43. Feng, Z.C. Handbook of Solid-State Lighting and LEDs; CRC Press: Boca Raton, FL, USA, 2017. 44. Horowitz, P.; Winfield Hill, W. The Art of Electronics, 3rd ed.; Cambridge University Press: Cambridge, UK, 2015. 45. Komine, T.; Nakagawa, M. Fundamental analysis for visible-light communication system using LED lights. IEEE Trans. Consum. Electron. 2004, 50, 100–107. [CrossRef] 46. Suzuki, K.; Asahi, K.; Watanabe, A. Basic study on receiving light signal by LED for bidirectional visible light communications. Electron. Commun. Jpn. 2015, 98, 1–9. [CrossRef] 47. Zuo, Y.; Zhang, J.; Qu, J. Power allocation optimization design for the quadrichromatic LED based VLC systems with illumination control. Crystals 2019, 9, 169. [CrossRef] 48. Monteiro, E.; Hranilovic, S. Constellation design for color-shift keying using interior point methods. In Proceedings of the 2012 IEEE Globecom Workshops, Anaheim, CA, USA, 3–7 December 2012; pp. 1224–1228. 49. Leccese, F.; Cagnetti, M.; Ferrone, A.; Pecora, A.; Maiolo, L. An infrared sensor Tx/Rx electronic card for aerospace applications. In Proceedings of the 2014 IEEE Metrology for Aerospace (MetroAeroSpace), Benevento, Italy, 29–30 May 2014; pp. 353–357. 50. Verma, S.K.; Madan, K.; Kaur, G. Future of visible light communication with li-fi technology: A review. Int. J. Innov. Res. Technol. 2015, 2, 82–85. 51. Dimitrov, S.; Haas, H. Principles of LED Light Communications: Towards Networked Li-Fi; Cambridge University Press: Cambridge, UK, 2015. 52. Masson, É.; Berbineau, M. Railway applications requiring broadband wireless communications. In Broadband Wireless Communications for Railway Applications; Springer: Cham, Switzerland, 2017. 53. Stepniak, G.; Kowalczyk, M.; Maksymiuk, L.; Siuzdak, J. Transmission beyond 100 Mbit/s using LED both as a transmitter and receiver. IEEE Photonics Technol. Lett. 2015, 27, 2067–2070. [CrossRef]

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