KELVIN PROBE EXAMINATION OF ORGANIC/METALLIC

SEMICONDUCTORS

______

A Thesis

Presented to

The Honors Tutorial College

Ohio University

______

In Partial Fulfillment

of the Requirements for Graduation

from the Honors Tutorial College

with the degree of

Bachelor of Science in Physics

______

By

Vincent Roberts

June 2012

Table of Contents

I. Introduction 4

A. An Energy Crisis 4

B. The Advent of the Solar Cell 6

II. Background and Theory 8

A. Solar Cell Technology: The Photoelectric . . . 8

B. Work Function and Charge Transfer . . . 11

C. Background on Kelvin Probe 15

D. “Seeing” at the Quantum Level 18

E. Principles of the Kelvin Method 19

III. Molecules and Preparation 25

A. Organic Molecule F4-TCNQ 25

B. Zinc Oxide 29

C. Sample Substrate Preparation 32

D. F4-TCNQ Preparation 34

IV. Data Collection 35

A. Kelvin Probe Preparation 35

B. Sample Probing 37

--2-- C. Data Collection Parameters 41

D. Discerning Between Good and Bad Data 42

V. Results and Conclusions 45

A. Discussion 45

B. Conclusions 50

C. Suggestion for Further Experimentation 54

Acknowledgements 56

References 57

--3-- I. Introduction

An Energy Crisis

Nano- and quantum technology are terms often used in science- fiction during the past several decades. They have become very much akin to “buzz” words of modern-day companies and technologies. The world is in the midst of a technological boom, yet it is also in a severe energy crisis. It’s no exaggeration to say that fossil fuels will be running short in the foreseeable future, and the call by environmentalists to save the planet have only added flames to the proverbial fire regarding the question of humanity’s sustainable energy dilemma. Green technology is now the hot field in the energy market, and solar power seems to be the next logical step in the crisis, but current solar panels operate at only less than 30% efficiency [1].

Traditionally, silicon has been the main source of solar cells.

However, silicon semiconductors seem to have reached their limit in the field, and now there is a push to examine new unique materials, utilizing the contemporary concept of charge transfer between organic and metallic semiconductor materials to perhaps find a more efficient solar cell candidate.

A prominent semiconducting-like organic material has come to the forefront: 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane, or,

--4-- simply, F4-TNCQ. Since it is a novel organic molecule, not much research has been conducted on it in combination with metallic semiconductors. There have been a few instances of work function being

[2] increased on ITO samples, increases of at least 0.6 eV . Since F4-TCNQ can cause an increase in work function in semiconductors, a preferable condition in solar cells, it is being tested on many other materials.

The focus of this paper is on the new organic material F4-TNCQ and its bonding properties to metallic semiconducting substrate, Zinc

Oxide (ZnO). F4-TNCQ has had interesting reactions with ITO in previous studies, successfully bonding and altering the work function of the materials; therefore, it can be postulated that it will have a similar effect with another metallic oxide such as ZnO. In order to make accurate measurements of the material work functions in an applicable setting, the sample was measured inside of a Kelvin Probe, which is able to make work function measurements at room temperature and at atmospheric pressure. Real solar cells must function in such conditions, so all data taken is done at a directly applicable level.

--5-- The Advent of the Solar Cell

While solar cells may seem like a relatively new innovation, specific to the green technology era, the photovoltaic effect (the working principle of the solar cell) was first discovered nearly 200 years ago by physicist Edmond Becquerel [1]. Extensive work on the photovoltaic effect was later done at the turn of the 20th Century by Albert Einstein, whose connection of quantum mechanics to work function of the photoelectric effect won him his only Nobel Prize. Much later, in the

1950’s, silicon became the most popular semiconducting material.

Silicon became the main source of diodes in electronics, most specifically in the form of photovoltaic (PV) diodes [1].

Ecological crisis of the 1990’ss and 2000’s, especially CO2-based global warming, have pushed sustainable energy science back towards solar power. Since the 1990’s, there has been continuing progress in the field of solar energy. Commercial prices in solar modules have shown a sustained average reduction of 7.5% per year, while global production of modules has increased on average 18% per year, with both expected to rise in the future [1]. However, despite trends of increases in production and lowering of manufacturing prices, the cost of installation of solar generating modules has remained high, approximately an order of magnitude higher than current prices of non-solar energies, e.g. hydro, nuclear, and fossil fuels [1].

--6-- Importantly, current solar cell technology is not a sufficient producer of energy. Lower-end solar modules used today are only efficient in energy conversion at about 15% [1]. As a result, more area, wiring, supports, and substrates are needed to provide enough energy for the modern energy driven way of life. Beyond the issue of low energy efficiency, there is a problematic artifact of production. While the use of

solar cells does not produce CO2 or other pollutants, their production generates pollutants and requires large amounts of energy during their manufacture [1]. This begs the question of how well the benefits of solar technology balance with their manufacturing on an ecological basis.

--7-- II. Background and Theory

Solar Cell Technology: The Photoelectric Effect

The foundation of solar cell technology began at the turn of the

20th century with physicist Albert Einstein. In his Nobel Prize winning research, he revealed the basis of the photoelectric effect using newly founded principles of quantum physics developed by Planck [3]. The photoelectric effect is the observation that when a metal surface is bombarded with incident light, electrons from the metal may be ejected

[3]. In order to explain this effect, Einstein focused on the localized packets of light energy, called photons, and their interactions with the metal. In this case, the photon packets were directly interacting with the electrons in the metal, giving them energy and causing them to be ejected from the metal surface [3]. In such a metal, electrons with energy to escape this well can be disassociated from the parent atom. This “escape energy” is known as the work function ϕ [3]. Energy must be conserved in this process, therefore the energy of an ejected electron by a photon must be:

E = hν – ϕ [3].

Whenever an electron is ejected by a photon, a positively charged

“hole” is left where the electron formerly was [3]. In a solar cell, the free electrons are captured via a voltage source, and the positive holes are

--8-- moved to the other side of the cell. This separation of charge creates a capacitor type structure in the solar material, with positive holes opposing the negatively charged electrons [1][4]. Such a capacitor builds up charge continuously as photons strike the surface, and thus energy is stored. Once the separation of holes and electrons is removed, there is a flow of charge, i.e. current, creating the wanted electricity [1][4].

Therefore, the basic solar cell is simply a semiconducting diode, a photovoltaic silicon diode being a common example [1]. A semiconducting material absorbs incident photons from a light source and then converts it to an electron-hole pair [1]. The main energy parameter comes from the work function of the solar material, specifically in the energy between band gaps, i.e. the valence and conduction bands [1][2]. Due to quantum mechanics, a photon with energy less than the band gap will not contribute to the photogeneration, but a photon with energy of the band gap or higher will contribute, with any extra energy rapidly lost [1]. Therefore, the maximum current density J is given by the flux of photons with energy greater than the band gap [1].

Similarly, current density J will decrease with an increasing energy gap.

However, not every photon that strikes the cell surface will contribute to charge separation of the electron-hole pairs. Solar cell efficiency is characterized by how many photons contribute to charge separation divided by the total number of incident photons. In theory

--9-- there can only be a maximum efficiency related to band gap and electron charge [1][4], yet nice solar films that have good quality can greatly enhance the efficiency. With many current models of solar cells, there are

only efficiencies up

to and approaching

30%, and most

materials such as

gallium arsenide,

indium phosphide,

Fig. 1: The standard solar cell uses and cadmium telluride are common silicon in order to reach energy efficiency 15-20%. much too costly for widespread application [1][4]. Some crystalline silicon cells reach an efficiency of 25%, but they are too sophisticated and too expensive for widespread industrial use [4]. Cheap modules that can be produced in large quantities rarely exceed an efficiency of 15% [1]. Therefore, there is a quest to find better materials to increase efficiencies. Research and use of silicon seems to have reached a limit, and more work is being done on other semiconductors. Organic semiconductors are also under scrutiny due to their easy availability and low cost [1][5].

--10-- Work Function and Charge Transfer in Organic

Semiconductors

By measuring the current over changes in voltage, dI/dV curves, energy spectra of the sample can be measured around its Fermi level at low temperatures [5]. Electron density will be higher in the two bands, supplying two energy peaks set before and after the Fermi energy [5][6].

For semiconductors, the highest occupied band is known as the valence band, while the lowest unoccupied band is called the conduction band

[7][8]. For molecular systems, the highest occupied molecular orbital and lowest unoccupied molecular orbital are known as the HOMO and

LUMO levels respectively [5][6]. Generally, the valence band (in semiconductors) and the HOMO (in molecular films) are located below the Fermi level, while the conduction band and LUMO are located above the Fermi level [5][9].

Combinations of inorganic and organic semiconductors, also known as hybrid inorganic-organic semiconductors can lead to interesting cases in terms of work function and the HOMO and LUMO energy levels [5]. This concept focuses mainly on the theory of molecular charge transfer between semiconductors. There are two subcategories for semiconductors: the donor N-type and the acceptor P-type hybrid semiconductors [8]. Interacting layers of N-type and P-type hybrid

--11-- semiconductors can create a multitude of molecular devices, most importantly diodes, organic light emitting diodes (OLED’s), and P-N- type transistors [10][11]. Much as its name implies, acceptors will take electrons from the donor type conductors. Sharing electrons, the transfer of charge, through molecular bonds will alter the structure of the molecules themselves, and will therefore also change the chemical structure and properties, creating something completely new from before

[10][11].

Starting with the deposition of the organic semiconductor onto a metallic semiconductor, the work function of both materials is expected to change drastically in comparison to prior deposition [11][12]. For both, there must be an alignment of the vacuum levels of energy, i.e. the conducting bands [2][11][12]. The electron injection barrier, the energy for an electron to jump to the conduction band, is given by the Schottky-Mott limit for the interface as:

ΦBe = Φm – EA

With ΦBe being the electron barrier, Φm as the metallic work function, and EA as the electron affinity [12][13]. This limit is purely for the electron injection; conversely there must be a limit for the holes created by the P- type semiconductor:

ΦBh = IE - Φm

--12-- With ΦBh as the hole barrier, and IE is the energy gap between the organic HOMO level and the newly aligned vacuum energy [12][13].

However, in experimentation it has been shown that the Schottky-

Mott theory is incorrect in its assumption of aligned vacuum levels of energy between the organic-metal interfaces [13][14]. Instead, there is an extra discrepancy between vacuum energies given by extra interactions between semiconductor surfaces. At the interface between the organic and metallic semiconductors, there is a dipole barrier separating the organic-metallic layers [13][14]. This dipole barrier creates an extra shift of energy Δ, the magnitude of which is completely dependent on the materials being used in the organic-metallic interface, i.e. different combinations of semiconductors will result in differing energy displacement Δ [13][14]. This leads to a correction of the electron and hole injection barrier equations, which can be fixed with determination of this dipole energy shift:

[13][14] ΦBe = Φm – EA – Δ and ΦBh = IE - Φm – Δ

This result will yield a better theoretical value than the previously described case of the Schottky-Mott interface [14].

--13--

Fig. 2: Corrected diagram with dipole barrier ∆. Importantly, the Fermi/vacuum energies align once the metallic sample (left) is bonded with the organic semiconductor (right).

--14-- Background on the Kelvin Probe

The Kelvin Probe, first introduced as the “Kelvin Method,” was introduced just prior to the turn of the 20th century in 1898 [13]. When Lord

Kelvin introduced the Kelvin Method in 1898, it was as a means to analyze the work function of metals and nonconducting materials [13]. However, technology of the times had not yet caught up, and the Kelvin Method would remain in theory and not practice. Through the 1900’s, many technological breakthroughs occurred in the sciences, but it wasn’t until the

quantum physics surge in

the mid 1980’s that the

Kelvin Method was

established in the modern

day Kelvin Probe [15].

Modern versions of the

Kelvin Probe work in

conjunction with an Fig. 3: Sir William Thomson, the Baron Kelvin of Largs. apparatus based upon the

1991 invention of the atomic force microscope and a variety of scanning- tunneling microscopes [15][16]. These advanced machines cannot only just characterize the surface and work function of a sample but also now acquire local nanometer resolution at the sample surface [16][17].

--15-- “Seeing” at the Quantum Level

In order to understand how the Kelvin Probe works, it is important to consider how scientists analyze materials. The question is, how can scientists “see” materials and examine them if they are too small to see with the naked eye? As it turns out, the concept of “seeing” is much more complex than what one would expect. By any definition of sight, light is involved directly through interacting photons with an object and the human eye.

For this problem, physicists had to be creative in the realms of quantum mechanics in order to bypass optical limitations. One such method involves using the Kelvin Probe, but, in many ways, the Kelvin

Probe is analogous to the scanning-tunneling microscope (STM). The STM exceeds the visible limitations by using the principle of quantum electron tunneling [18]. Quantum tunneling comes directly from the Schrodinger model of electrons, i.e. that electrons can be thought of wave packets rather than point charges [3][8]. If two conducting materials are brought into a proximity that is shorter than the wavelength of an electron and a potential difference is applied, there will be a flow of electrons between the two conductors [3][8]. In practice, an extremely sharp metal tip, such as a

Tungsten (W) tip, is used along with a conducting sample substrate, such as gold (Au) or silver (Ag) [18]. Once a voltage is applied along the tip and the sample, a current flow can be measured from tip to sample or vice versa

--16-- depending on the voltage. The current is inversely proportional to the tip height (z): smaller z = greater current, bigger z = smaller current. As the tip is moved along the sample, differing current densities are recorded along with tip position. In this fashion, a topographical map of the sample surface can be produced with atomic level resolution [18].

While the STM utilizes currents to receive information and “see” the sample surface, the Kelvin Probe acts in a somewhat similar manner using potential difference, i.e. voltages. Again like the STM, the probe and sample are not in contact, although the probe is also brought close to the sample. However, there is no electron flow between the sample and the tip in a Kelvin Probe. Instead, by setting a voltage across the two, the tip and sample form a capacitor type structure [2][13]. The tip can be moved along the surface (much as the STM) to profile the surface of the sample. This includes mapping the surface potential of the sample, examining qualities such as purity and the doping factor [15][16]. However, in modern applications, the main quality to be examined is the work function of the material in question.

--17--

Fig. 4: a) Large scale STM apparatus, along with b) the W tip moving across the sample surface.

--18-- Principles of the Kelvin Method

Since its genesis in 1898, the principle of the Kelvin Method has had relatively few modifications. As discussed before, the tip and sample surfaces are brought into extremely close contact at a distance d. Before electrical contact is made (switch 1, SW1 is open) between the two

surfaces, each occupies the lowest energy state at the Fermi level (εf1 and

[2][13] εf2) . The work functions of the samples (φ1 and φ2) are the energy

differences between the Fermi levels and the vacuum energy Evac. Once a voltage is applied, and the sample/tip are in electrical contact after SW1 is closed, there is an electron flow from the lower work function to the higher

[2][13] . In the case where φ2 < φ1, there will be a flow of electrons from surface 2 to surface 1. From the flow of charge between surfaces, there will be a build up of charge on the plates: since electrons move to surface 1, there will be a build up of negative charge on surface 1 and a build up of positive charge on surface 2. Thus, a capacitor state is formed between the

two surfaces named the Kelvin capacitance CK along with the voltage

[2] across the capacitor of Vc . The capacitor voltage is directly related to the work functions of the two materials as:

Vc = 1/e (φ2 – φ1)

Here, e is the fundamental charge of an electron [2]. With both the voltage

and capacitance known (Ck and Vc), the built up surface charge can be

--19-- readily evaluated by the fundamental capacitance equation of: Qs = VcCk

[2][3].

Fig. 5Before, during, and after SW1 is closed. Before (a), there is no flow of electrons, and each remains in its Fermi state. As the SW1 is closed

(b) there is a flow of electrons and a build up of charge Qs on the surfaces. Once a backing voltage is applied (c), the charges cancel, and the difference in work function Δφ can be measured.

If a measureable and external backing voltage Vb is applied to the

circuit that flows opposite to Vc, the charge on each side of the surfaces can be cancelled out:

Qs = (Vc + Vb)Ck

[2][13] Cancellation of charge occurs only if Vc = -Vb . Since the backing voltage is a measureable quantity, a cancellation of charge means that the

difference in work function Δφ can be directly evaluated: when Qs is cancelled, Vc is known to be –Vb (a given value), then Δφ can be solved:

[2][13] Vc = 1/e (φ2 – φ1)

--20-- However, while the Kelvin Method can directly measure Δφ, it creates a one-time-only measurement due to the surfaces having become charged [2]. Before another measurement can be made, both surfaces must dissipate their built up charge. Setting and resetting the probe in order to gather new data points is cumbersome and time consuming, yet this drawback can be easily circumvented by use of a vibrating probe, creating a vibrating capacitor [15]. Modern Kelvin Probes make use of an alternating- current (AC) voltage source in order to create a varying probe tip and capacitance given by:

C = Q/V = ε0εrA/d

Where ε0 is the permittivity of free space, εr is the relative permittivity of the material, A is the surface area, and d is the distance between the surfaces. [2][13] Via this capacitance equation, it is quite clear that as the separation of the surfaces increases, the capacitance must increase. Since

the charge Qs must remain constant on the surface, then the voltage VC must increase as well.

--21-- During the oscillations, the voltage change ΔV due to the changing tip height is recorded. With a high modulation index ε ≈ 0.7, the signal will

[2] contain sharp peaks and troughs that are asymmetric in time due to Ck .

Over time, a repetitive pattern is formed as the tip vibrates in periodic

motion giving another voltage value, namely peak-to-peak voltage Vptp that is given by:

Vptp = (ΔV – Vb) RC0ωesin(ωt + θ)

Fig. 6: Signal voltage changes over time as the vibrating tip oscillates over the sample surface. As the tip height increases, the voltage must increase as well. As the tip height decreases, the voltage must decrease. Over time, the signal becomes periodic.

With ΔV as the voltage difference between tip/sampls, R being the circuit

resistance, C0 being the average capacitance of Ck, ω as the frequency of the vibrating tip, t as time, and θ as the phase angle [13]. When measuring

Vptp, it is considered negative if a trough is seen before a peak: in the above

[2] image, the Vptp would be considered negative . The backing potential is

--22-- set to scan a range of voltage values, which is then plotted against the peak-

to-peak voltage. If Vptp is plotted versus Vb, a straight line is formed, and when the Vptp = 0, then the corresponding Vb is equal and opposite to the

[2][13] Δφ (via Vc = -Vb) .

Fig. 7: A plot of Vptp (y) vs Vb (x). The linear plot shows a balanced circuit once Vptp = 0. At this point, the backing voltage is equal to Vc allowing for Δφ to be calculated.

The method of plotting peak-to-peak voltage during interval steps of backing voltage is known as Off-Null signal detection [2]. In most cases of

Kelvin Probe measurements, Vb is measured from a range of -5V to 5V.

This process is known as the “Baikie Method,” named physicist

Iain Baikie [13]. Kelvin Probe Technologies has a unique system that allows for many measurements and remeasurements on a single sample surface for different times and different tip heights. The KPT Kelvin Probe has software in the form of a “Tracking Modulus,” a useful system that allows for a gradient value to be set such that no measurement will occur outside of a tolerance range, which allows for more precise measurements [13]. The

--23-- Baikie Method also uses extrapolation for finding Vptp = 0 in order to work

[16] with a high signal/noise (S/N) ratio . Values extracted for Vb that set Vptp

= 0 has a minimum S/N value, with noise creating an offset voltage that

can nullify results [16].

--24-- III. Molecules and Preparation

Organic Molecule F4-TCNQ

Organic molecules have become increasingly relevant in the world of condensed matter and quantum physics. Many organic samples have proven to have quite useful properties as semiconducting materials that other inorganic and metallic materials do not. Chiefly, experimentation has focused on P-N-type junction diodes, organic light emitting diodes (OLED’s), transistors, and other nanoscale electronics

[5][10][11]. In the search for cheap and efficient semiconductors for green solar energy sources, the majority of quantum physicists have begun research in the organic-metallic hybrid semiconductor research. The prospect of finding highly efficient materials is one that is defining the role of the quantum realm of science.

Recently, a new molecule entered the organic semiconducting

race: F4-TCNQ, or 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano- quinodimethane, a strong π-electron acceptor [2][10]. At this point in time,

there has been little research done with F4-TCNQ for solar cell technology and application. The experimental process is still in its

earliest stages, and F4-TCNQ has not made a large impact as of yet. Now, there is a large push to experiment with F4-TCNQ on metallic

--25-- semiconducting materials, mostly oxides such as Indium Tin Oxide

(ITO) [2][10].

[2] F4-TCNQ works well as a π-electron donor in semiconductors .

This is common for organic materials, small molecules, and polymers; all organic semiconductors [10]. The terminology of π-electron bonding is formed from the basis of the P-orbitals, P-orbital is replaced by the Greek symbol π. These π-bonds are covalent bonds formed from the P-quantum orbital state of atoms; overlapping orbitals cause a bonding attraction between atoms and molecules [3]. Due to orbital symmetry, the π-bonding of the P-orbital lobes aligns in “parallel” rather than P-lobe-tip to P-lobe- tip. This gives a planar type bond, face-to-face [2]. By the structure of the bonding, this allows no rotation between bonding molecules, at least without breaking the π-bonds and disassociating the bonded structures.

Due to this orientation, the faces are fixed in position unless sufficient energy is applied to the bond. Aside from the P-orbital, there is a weakly interacting D-orbital bond that contributes slightly as well [2]. For the case

of the F4-TCNQ molecule, the π-electron system is a bonding of the

2 overlapping pz-orbitals of the sp -hybridized carbon atoms centrally located in the molecule [2][10].

The atomic structure of F4-TCNQ has been further studied in a few low temperature environments. Under the conditions of an ultra high vacuum and low temperature, the scanning-tunneling microscope gives a

--26-- clear picture of F4-TCNQ on a sample substrate. Low temperature experiments such as these allow for a closer look into the molecule near

Fig. 8: F4-TCNQ examined at UHV liquid He temperatures: a)

structure of F4-TCNQ, b) deposited layer of molecule on 6T surface, c) alignment of molecule on surface, d) HOMO and

LUMO peaks of F4- TCNQ, structure of peaks denotes it as a semiconductor [10].

low-temperature ideal conditions, allowing for examination near the

molecule’s Fermi level. It was found that F4-TCNQ forms self-assembled clusters two-dimensionally on the Au(111) surface under cell lattice parameters a = 0.87 ± 0.07 nm and b = 1.22 ± 0.08 nm and α = 73 ± 8º

[18]. Single molecules were mapped via dI/dV spectra, showing that the highest occupied orbital HOMO and the HOMO-1 peak at -1.0 V and -

1.4 V respectively before the 0 V Fermi level of the system [18]. Lowest unoccupied orbital LUMO was found at +0.6 V along with two excitation shoulders at -0.22 and +0.22 V [18].

A similar type experiment with F4-TCNQ was done on indium tin oxide (ITO) under room-temperature (300K) conditions [2][11]. Here, the

Kelvin Probe data shows a maximum change in work function of about

--27-- [2] 400 mV after an exposure to F4-TNCQ for a period of two days . Such conditions of exposure imply that there was a complete monolayer on the

ITO surface. Since such work function changes were seen on a metal- oxide, it is expected that similar types of materials with metallic-oxide properties will have a similar effect. Therefore the common material ZnO proves to be an obvious choice for further testing.

--28-- Zinc Oxide

Zinc Oxide (ZnO) has been a common compound used for thousands of years throughout human history [19]. While ZnO may have deep roots in human history and industry, there was little research done on the material until the turn of the 20th century. There was an outburst of research done in the 1970s and ‘80s, which began to fade due to the impossibility of simultaneously doping the material with P- and N-type doping [20]. Without such doping, it proved to be an ill choice for optoelectrics and other emerging applications of quantum physics. Much of the early research done on ZnO focuses on synthesis, growth, doping, transport, band structures, luminescence, and surface polarization [19][20].

However, there was another swell of ZnO research beginning in the mid-1990s followed by continuing research in the early through mid-

2000s. The enthusiasm for ZnO was based on five main hopes for the material: 1) it could be used as a material for light emitting or laser diodes in replacement of GaN-based systems; 2) a radiation shielding material for electronics used in radiation environments; 3) a material for electronic circuits that would be transparent in the visible spectrum; 4) a diluted or ferromagnetic material that could be doped for semiconductor spintronics; 5) a highly conducting oxide that could be doped as a cheaper alternative to indium tin oxide [20]. For the purposes of this paper, it is the 5th goal that will be thoroughly examined, with ITO being

--29-- replaced with ZnO in the pairing with organic semiconductor F4-TCNQ.

For thin film deposition, many laboratories follow a simple method of close-distance evaporation of ZnO onto Al2O3 (sapphire) substrates, which can then be used in a variety of apparatuses, including the Kelvin Probe [20].The structure of the crystalline unit cell of

ZnO is complex: the Fig. 9: The complex hexagonal wurtzite structure that comprises the hexagonal wurtzite-type ZnO crystal. structure [21]. It has a polar hexagonal axis along the z-axis [21]. One zinc ion is surrounded tetrahedrally by four oxygen atoms that are then further surrounded by zinc. The unit cell ratio c/a of the translation vectors come to about 1.60, deviating slightly from the ideal c/a value of (8/3)1/2 ≈ 1.633 [21]. As a type IIb-VI semiconductor, ZnO contrasts to others, in that it exists primarily in the wurtzite-type structure rather than in the blend of the cubic zinc and the hexagonal wurtzite [20]. This type of tetrahedral coordinated structure is characteristic for covalent chemical binding with sp3 hybridization, although ZnO also shows a substantial ionic bonding component [20][21]. This fraction of covalent-ionic bonding, the LUMO

--30-- state is formed from the 4s levels of Zn2+ and the HOMO levels at the 2p level of O2- [21]. At low temperatures, the band gap between conduction and valence bands at low temperatures is about 3.5 eV [21].

The structure of the tetrahedral Zn2+ and O2- allows for a unique structure to be formed with stacking alternating planes of zinc and oxygen surfaces. There are four low-index surfaces of value: the non- polar (1 0 1 0) and (1 1 2 0) surfaces, and the polar (0 0 0 1)-Zn and (0 0

0 1)-O surfaces [21]. The polar surfaces can be thought of as the termination of the unit cell with either zinc or oxygen ions. These two surfaces prove to be the most interesting of the four surfaces: the Zn and

O-termination sides have differing physical and chemical properties from one another. Also, theoretically, the polar ion surfaces should have an extra instability due to a non-zero dipole moment perpendicular to the surface [21]. However, experiments have shown that such surfaces of ZnO are, indeed, stable in contrast to what was previously thought. Such stability could be theoretically achieved by either creation of new surface states, removal of surface atoms, or the adsorption of impurities [21].

One of the most crucial applications of ZnO will be in the solar cell market, as it acts as a useful front contact for solar cells while avoiding shadowing effects [21].

--31-- Sample Substrate Preparation

The sample substrates consisted of two 1 cm2 sapphire glasses covered on one side by the ZnO (one with O-terminated and the other

Zn-terminated) via an evaporation process. ZnO deposition was determined to be successful after an examination by x-ray photoemission spectroscopy (XPS), which showed strong photon energy peaks from the zinc and oxygen spectra. In order to determine which sides of the sample substrates that had the ZnO layer, small scratches were marked in the upper right hand corner so that the samples would not be flipped in the

Kelvin Probe.

An extensive cleaning procedure was followed by every new series of data acquisition. The process was done in order to eliminate any organic or dust particles that may have bonded or landed on the ZnO substrate layer. Excesses of extraneous particles could change the work function of the material, giving poor results on the Kelvin Probe, or

prevent a good bonding of organic F4-TCNQ onto the substrate. The standard cleaning process is as follows:

1) Washing in distilled water

2) Ultrasonic washing in chloroform bath (5min)

3) Washing in distilled water

4) Bathing in 0.1 molar citric acid (5)

5) Washing in distilled water

--32-- 6) Air drying with inert nitrogen gas

After washing, the samples were placed into an airtight plastic case in order to prevent any more build up of dirt and to prevent any extra

exposure. In terms of the

citric acid process, a solution

was previously used with 2%

concentrated HCl. However,

this was deemed to be too

strong of an acid, and it was

Fig. 11: The prepared and clean strong enough to remove the ZnO sample. The ZnO was deposited onto a glass surface and ZnO substrate layer and etched for recognition. destroy the sample even after a

low exposure time of < 5sec.

--33-- F4-TNCQ Preparation

A solution of organic molecule F4-TCNQ was prepared using 3.3 mg of F4-TCNQ with 100 mL of solvent tetrahyrdoflourine (THF). This created a solution of 0.12 mmol/L F4-TCNQ in the THF. The solvent

THF was used due to the similar qualities of chloroform, which has been

[2] used in previous experiments with ITO . The solution of F4-TCNQ should be a dull yellow color with no distinct smells. As a precaution, the solution was placed underneath a fume hood for health safety. The solution was also capped and isolated while not in use so that stray particles couldn’t contaminate the solution.

Fig. 12: The F4- TCNQ and tetrahyrofluorine solution. A good solution is noted by its dull yellow coloration.

--34-- IV. Data Collection

Kelvin Probe Preparation

Before a sample can be measured with the Kelvin Probe, an optimal voltage signal is needed, which can be fine tuned by adjusting the parameters on the probe, most importantly the tip height z [2][13]. A good tip height is evident by a smooth sine- wave-esque signal, with only one peak and one trough [2]. A Fig. 13: Tip is brought over the sample, as seen on the oscilloscope. Tip oscillation (red) is plotted signal with more along with peak-to-peak voltage. than one peak and trough leads to poor measurement and an erroneous value of work function. Also, it is important that the peak and dip of the signal are grouped tightly together and not too close to the edges of the data array, which will lead to large error values. These parameters can be achieved with combinations of probe oscillation frequency, amplitude, trigger, and rate, all of which in the best combination register a clear signal of a clean single peak and trough.

--35-- The main parameter that will have the greatest impact on a successful work function measurement is the tip-sample distance, or the tip height. This value can be found and maintained during measurement with manual tip amplitude and the GradReq value. The GradReq value is used to keep the tip height at a constant value, within the limits of an input value of gradient. Values that are measured outside this gradient value are discarded as poor measurements.

--36-- Sample Probing

Starting with a cleaned sample substrate and a prepared F4-TCNQ solution, the sample is ready for examination via the Kelvin Method. The main procedural process follows a simple time dependent experiment; the

ZnO sample substrate is to be exposed to the F4-TCNQ solution until a maximum change in work function is found. At that point, it can be

assumed that there is a complete monolayer of F4-TCNQ on the ZnO surface. At this point of maximum work function change, the sample is completely saturated.

However, data points output of the Kelvin Probe is not the exact work function of the material; rather, it is the difference in work function between sample and probe tip, ∆ϕ. Therefore, before the ZnO + F4-

TCNQ sample is tested, a reference sample with known work function must be used. For this experiment, a vacuum cleaned Au sample was used with a known work function of 5.1 eV. The output work function value will be of the form Δϕ = Au – Tip, and this measured value can be used to find the work function of the Kelvin Probe’s vibrating tip. Hence, when the sample is examined, the new output will be Δϕ´ = ZnO – Tip, and with the measured Δϕ´ and Tip work function known, the sample work function can be readily determined.

Immediately after sample cleaning, probing should begin.

Decreasing the amount of time between cleaning and probing reduces

--37-- any events of contamination on the sample surface. Probing with the

Kelvin Probe follows a short method of repetition:

1) Put the clean sample in the Kelvin Probe, making sure that

all contacts are in place.

2) Turn on the Probe, and reduce tip height until there is a

good tip-to-sample signal on the oscilloscope and

computer.

3) Develop a baseline scan with the completely clean (only

ZnO) sample.

4) Place the ZnO sample in the F4-TCNQ and THF solution

for one time interval cycle.

5) Wash the sample with THF.

6) Air-dry the sample with the nitrogen gas.

7) Scan again with the Kelvin Probe with the new F4-TCNQ

added sample, measuring work function with the LED

on/off inside the Probe.

8) Continue these steps with increasing time intervals in the

F4-TCNQ solution until a saturation level is found.

Evaluation of the saturation levels of the ZnO + F4-TCNQ combination is the ultimate endgame of the experiment. Experiments on the saturation levels of ITO sample substrates with F4-TCNQ have long

--38-- exposure times, up to many hours. Therefore it can be expected that a similar length of time may be required before finding saturation.

In measuring the ZnO samples with organic F4-TCNQ, two

effects will be

simultaneously

measured along with

saturation. As

mentioned before,

there is a slight

photovoltaic effect Fig. 14: ZnO + F4-TCNQ before Kelvin Probe measurement. The tip is brought over the sample until a capacitor structure is seen by the active formed. LED inside the interior of the Kelvin Probe. This leads to some changes in work function of the material (energy of the LED photons contribute to the required energy gap of the material), but just how much remains a question.

Therefore, this voltaic effect are measured in contrast to without any

LED contribution. How the LED affects the ZnO once F4-TCNQ is applied is currently unknown. There has been some discussion over the structural and therefore electronic properties of the two types of polarized

ZnO crystal edges, namely the zinc- and oxygen-terminated edges of the crystalline structure of ZnO [21]. It is theorized that the different edges will interact differently with the addition of F4-TCNQ due to the different

--39-- electronic properties of zinc vs. oxygen. The effect should be undeniably evident in terms of the combined work function, and each should show a different change in work function from before. Measuring the differences in work function at saturation is important for future applications of ZnO and F4-TCNQ.

--40-- Data Collection Parameters

During measurement on the KP-Technologies LTD Kelvin Probe, parameters for operation were set in order to get the optimum signal.

Such a signal is smooth and sine-like, with a peak and trough clearly showing on a peak-to-peak voltage plot over time. The parameters used for the probe setup and data acquisition are amplitude: 3.8 mm, frequency: 80.9 Hz, backing voltage: 5 V, with the measurement gain of

4.

Fig. 15: A good signal created by the above input parameters. Cleary, it is a smooth function with the coveted “one peak and trough,” which is integral in acquiring good and accurate data.

--41-- Discerning Between Good and Bad Data

The realistic settings of the Kelvin Probe at room temperature and standard atmospheric pressures pose some inherent measurement problems. While such a setting will provide the most accurate results for real world settings, error easily arises in the form of contamination.

While cleaning the sample removes most imperfections on the ZnO sample substrate, it is inevitable that some minute dust, water, and organic molecules will contaminate the sample even while under the probe. These errors can be eliminated inside a UHV chamber, yet such values would be only ideal, and results would not necessarily be applicable in a real setting.

Effects of contamination are quite noticeable in the work function plots given by the Kelvin Probe. Ideally, work function data points would remain fixed around a certain level; the average work function measured would be the actual work function of the material. This is not necessarily the case with contamination. Usually, most of the contamination on the surface will be negligible, and any light contamination residue is actually blown off the surface of the sample due to an air pressure created by the vibrating tip. As such, any traces of water may also evaporate, leaving the sample mostly clean. Normally, these effects will be seen in the first couple of data points over the first minute or so of data acquisition; evident by rapidly rising or falling data points. After this threshold, data

--42-- will begin to concentrate around the correct value of work function. In such cases, the “leveled” off area of the data plot contains the wanted work function, and the initial values can be safely ignored. Such a data set can be considered as a “good” data run. However, some sets do not level off in the time frame of a data acquisition run; yet there the plot is clearly reaching a limit on work function. The data, still, can be useful in this regard. Simply fitting the data to a decaying function can give correct values of work function. As such, decaying work function plots are considered “good” data as well.

However, there are some data plots that seem to never level off in a reasonable time frame. Even for extensive amounts of time in the

Kelvin Probe, there is never any evidence of a final maximum or minimum work function value, and no exponentially decaying function can be fit to the data. Such a plot is considered “bad,” and has to be taken again. Interestingly, these non-equalizing plots are seen mostly during data runs when the Kelvin Probe LED was on inside the Probe.

Normally, dark plots showed no such effect, therefore it may be assumed that this is due to some photovoltaic effect, with photons from the LED

source affecting the work function of the combined ZnO + F4-TCNQ system.

--43--

Fig. 16: An example of a “good” data run. While the plot looks to be approaching a maximum value towards the end, the actual work function can be extrapolated by fitting the plot to a decaying curve.

T h

Fig. 17: An example of a “bad” data run. A minimum value of work function is not seen, and no value can be extracted with a fitted curve. Such a plot is thrown out of the final result.

--44-- V. Results and Conclusions

Discussion

The ultimate goal of the ZnO + F4-TCNQ experiment was two- fold. Work function dependence on exposure time to the F4-TCNQ and

THF solution was measured in order to find 1) the work function at maximum exposure and equilibrium, and 2) to examine on what time scale it took for the work function to reach this equilibrium state.

Secondary to these two primary goals, the photovoltaic response due to the Kelvin Probe LED was measured in order to see how a light source would affect the sample. Theoretically, energy of the photons emitted from the LED light will alter the work function of the material; photons supplying some of the required energy to band gap between the valence and conducting bands of the material. Therefore, samples with the LED on will have a smaller work function than samples not exposed.

Another goal of the project was to examine the differences between the two crystalline structures of ZnO along with their exposure

to F4-TCNQ. This was the difference of the bonding atoms of the unit wurtzite crystal: one sample with Zn-terminated and one with O- terminated. Different bonding atoms should result in different bond types and new electronic structures. Since electronic structures of atoms and

--45-- molecules dictate their properties, the two different terminated samples will have unique differences in work function.

O-term 1 min Light Off 4800 4750 4700 4650 4600 4550 4500 Work Function (mV) Work 0 50 100 150 200 250 300 Data Point

O-term 1 min Light On 4560 4550 4540 4530 4520

Work Function (mV) 4510 0 50 100 150 200 250 300 Data Point

Fig.18 & 19: Sample data collection of an Oxygen-terminated sample. There is a clear difference between the light being on/off. The “off” sample has a slow increase over time, and a maximum can be established. The “on” sample has a large increase followed by relative stability; another good data run.

--46-- Zn-Term 1 min Light On 4900

4850

4800

4750

Work Function (mV) Work 4700 0 50 100 150 200 250 300 Data Point

Zn-term 1 min Light Off 4820 4810 4800 4790 4780 4770 4760 Work Function (mV) Work 0 50 100 150 200 250 300 Data Point

Fig. 20 & 21: Sample data collection of Zinc-terminated sample. LED on/off seems to switch between increase and decrease of work function. The “off” run looks to begin stabilizing at around 4800 mV, while “on” starts leveling at 4760 mV. These samples’ max/mins must be extrapolated with a fit curve.

However, what is really interesting from these cases is the final result from each terminated series. The short time exposure plots do not paint the entire picture, but rather serve as examples from what the output will initially look like after analysis. Nonetheless, it appears from the above examples that the O-terminated sample seems to be more stable in work function than the Zn-terminated counterpart.

--47-- ZnO(Zn-Term) + F4-TCNQ Light On vs. Off 4950 4900 4850 4800 4750 4700 Light On Light Off

Work function (mV) Work 4650 4600 4550 -300 700 1700 2700 3700 4700 5700 F4-TCNQ Exposure Time (sec)

ZnO(O-Term) + F4-TCNQ, Light On vs. Off 4800

4750

4700

4650

4600 Light On 4550 Light Off Work Function (mV) Work 4500

4450 -300 700 1700 2700 3700 4700 5700 F4-TCNQ Exposure Time (sec)

Fig. 22 & 23: Final plots, showing both terminated samples along with differences between LED off/on. Both terminated samples show a similar shape in saturation over exposure times to F4-TCNQ. Also, both seem to reach eventual saturation times, and each has a considerable difference in LED work functions.

--48-- Both terminated samples follow a similar pattern. At short time intervals, on the order of a few minutes, there is a rapid increase of work function for both light on and off samples. However, by the 700 second mark, there is a notable drop off in work function gain, with only a few mV added from the previous measure. At this same time interval, a maximum difference in work function between light on and off samples is attained and sustained up to the last measurement. Again, for both Zn- and O-terminated samples, there is a consistent ~100 mV difference, with the light off samples having the greater work function. While the difference in work function may be on the order of 100 mV for each sample, there is still a difference in overall work function for both.

According to the final overall plots, the Zn-terminated sample had the largest increase in work function, up to about 4900 mV versus the O- terminated sample that reached only on the order of 4750 mV (with light off for each). The Zn-terminated sample also had the greatest overall increase in work function Δϕ from before and after the inclusion of the

F4-TCNQ molecule, with Zn-term having Δϕ ≈ 350 mV and O-term having Δϕ ≈ 250 mV.

--49-- Conclusions

The goal of this project was to determine work function changes due to charge transfers between the metallic ZnO metallic sample substrate along with the added organic semiconductor F4-TCNQ. This involved measuring work function variations after increasing time frames of exposure to the organic F4-TCNQ, with the hypothesis that at some time interval of exposure, the ZnO would be completely saturated with a monolayer of the molecule on its surface. However, it was deemed necessary to measure these work function changes along the two ionic cases of ZnO, which are determined by its unique crystalline structure.

On one orientation, the bonding surface plane is dominated by only zinc atoms, whereas another orientation of the ZnO crystal has the oxygen atoms facing outwards on the bonding plane. Due to the fact that the number of electrons in a system set all electronic and material properties of the system, it was hypothesized that the different orientations would have different overall reactions with the F4-TCNQ, ultimately regarding different effects on work function changes.

By a method first laid down by Lord Kelvin in the 1800’s, the change in work function was measured using a KP-Technologies LTD

Kelvin Probe, an apparatus that measures a given sample’s work function under real-world conditions, i.e. standard atmospheric pressure and temperature. Such parameters over ideal conditions, ultra high vacuum

--50-- and liquid He temperatures near the Fermi level, were deemed practical, as any device in the future utilizing ZnO + F4-TNCQ must operate under such conditions.

Further, another variable was simultaneously measured concerning the ZnO + F4-TCNQ was the added LED interacting, or the photovoltaic effect added by an LED located inside the Kelvin Probe.

Any effect given by a light source may be interesting in the future, especially for applications in the solar cell field of materials. Ideally, the packets of energy represented by the photons would affect the work function, actually decreasing its value since it is added energy that allows for the electrons to jump between the valence and conducting bands of energy.

As the final results show, there was indeed successful bonding, and therefore charge transfer, between the metallic semiconductor ZnO and organic semiconductor F4-TCNQ. Without such a bond, the work function would remain constant through time, yet such a change in work function over exposure times is direct evidence of a successful combination between the two. Also, it can also be seen that each terminated sample of ZnO had a saturation level attained at around 700 seconds, and there is little increase after, denoting a saturation of the surface. Both the light on and off experiments saturated at this same time interval, although there is a marked difference between the two. As

--51-- expected, the light on experiment had a significantly smaller increase in work function with the inclusion of F4-TCNQ, and the light off experiments seemed to be greater by a matter on the order of 100 mV for both the Zn- and O-terminated samples. Consistently, it seems that the photovoltaic effect only affects this regime, and most notably at the saturation levels. For small exposure times to F4-TCNQ, the ZnO has little difference between light on/off, yet the effect becomes much more pronounced after a few minutes of exposure. Therefore, it seems reasonable to assume that the F4-TCNQ makes the ZnO much more responsive to the effect of a light source, a facet that might prove important in solar cell research.

However, there are, indeed, some differences concerning the Zn- terminated and O-terminated samples of ZnO. Firstly, the Zn-terminated sample had a much higher overall light-off work function than the oxygen sample. At ~4900 mV, the Zn-term sample had a greater work function by a difference of about 150 mV. Not only did the Zn-term sample have a higher overall work function than the oxygen, it had a greater overall increase in work function Δϕ. From before and after the inclusion of the F4-TCNQ molecule, the Zn-term had a Δϕ ≈ 350 mV and

O-term with Δϕ ≈ 250 mV. Therefore, it seems reasonable to assume that the zinc sample was much more reactive than the oxygen sample, on the order of 100 mV, or 0.1 V. Due to it’s greater reactivity, the Zn-term

--52-- sample may be much more useful in following experiments, where its reactivity with F4-TCNQ can be better fine-tuned for different values of work function. However, on the other hand, the oxygen-terminated sample seems to be much more stable overall, which may allow for less sensitive applications.

--53-- Suggestion for Further Experimentation

It is important to stress that these experiments with ZnO and the organic F4-TCNQ are in the very beginning stages. Such a process measuring the work function is an initial foray into what could perhaps become a greater field of ZnO + F4-TCNQ in the realm of organic- metallic semiconductors. Fleshing out just how the materials react adds a further boon to the scientific process. ZnO + F4-TCNQ has had little inspection as of yet, and it would be interesting to put the samples together under ideal conditions, such has been done with ITO and F4-

TCNQ in a scanning-tunneling microscope.

Of course, once the base of experimentation is established, it would be time to advance studies into possible applications in solar cell technology. This could be a literal gold mine for green technology and solar science. With commercial solar cells reaching only on the order of

20% efficiency, finding a new material that works at a higher efficiency for a lower cost would put it way above a competitive level in the green energy market. Already, the race is under way, with many interesting quantum materials, many focusing on quantum silicon crystals and

Carbon(60) substrates. ZnO + F4-TCNQ proves to be an enticing material, with ZnO being a very common and cheap material. In any case, any application forthwith could be a very lucrative endeavor.

--54-- As such, this experiment proves to be an all too crucial stepping stone into a greater field of experimentation. All great advances in technology had to start from the ground up, and so is the case of ZnO and

F4-TCNQ. The next few years will, indeed, be quite exciting for the case of small-scale semiconductors and solar cell technology, and it may be that the molecules investigated in this experiment become critical to the field.

--55--

Acknowledgements

The author would like to thank many people that made this research opportunity possible. Firstly, thesis advisor Dr. Saw Hla of Ohio

Univeristy and the collaborating SPIRE program for his time and help as an advisor and introducing the wonders of the quantum world.

Administrative assistant Julie Goettge of SPIRE, who helped to make a trip to Humboldt University in Berlin possible. Dr. Norbert Kock of the

Supramolecular Systems of Humboldt University for his hospitality and mentorship as the leading professor of the German labs. Post doctorate

Raphael Schlesinger for his work with the Kelvin Probe and ORIGIN software extrapolation techniques. Director of studies Dr. David Drabold for overseeing the entirety of the thesis project.

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--58--