Free-Standing Self-Assemblies of Gallium Nitride Nanoparticles: a Review

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Review Free-Standing Self-Assemblies of Gallium Nitride Nanoparticles: A Review

Yucheng Lan 1,*, Jianye Li 2, Winnie Wong-Ng 3, Rola M. Derbeshi 1, Jiang Li 4 and Abdellah Lisﬁ 1

1 Department of Physics and Engineering Physics, Morgan State University, Baltimore, MD 21251, USA; [email protected] (R.M.D.); Abdellah.Lisﬁ@morgan.edu (A.L.) 2 Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA; [email protected] 3 Materials Science Measurement Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA; [email protected] 4 Department of Civil Engineering, Morgan State University, Baltimore, MD 21251, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-443-885-3752

Academic Editor: Massimo Mastrangeli Received: 6 April 2016; Accepted: 12 July 2016; Published: 23 August 2016

Abstract: Gallium nitride (GaN) is an III-V semiconductor with a direct band-gap of 3.4 eV. GaN has important potentials in white light-emitting diodes, blue lasers, and ﬁeld effect transistors because of its super thermal stability and excellent optical properties, playing main roles in future lighting to reduce energy cost and sensors to resist radiations. GaN nanomaterials inherit bulk properties of the compound while possess novel photoelectric properties of nanomaterials. The review focuses on self-assemblies of GaN nanoparticles without templates, growth mechanisms of self-assemblies, and potential applications of the assembled nanostructures on renewable energy.

Keywords: self-assembly; nanoparticles; Gallium nitride (GaN); renewable energy; review

1. Introduction Gallium nitride (GaN) based semiconductors have attracted great attentions since the 1990s [1–4]. GaN is a binary III–V direct band-gap semiconductor. Wurtize GaN has a space group of P63mc with lattice constants a = 3.1891(1) Å and c = 5.1853(3) Å [5]. Its wide band gap of 3.4 eV [6,7] affords the nitride special properties for applications in optoelectronic, high-power and high-frequency devices, high-temperature microelectronic devices, and bright lighting sources. For example, (1) hexagonal GaN crystalline films have been fabricated as blue light emitting diodes (LEDs) [3,4,8–10] because of its special optical emission [11,12]. The alloyed InGaN- and AlGaN-based LEDs can emit colorful light from red to ultra-violet [9]; (2) GaN films have been employed to make violet (about 405 nm) laser diodes (LDs) [3,4], without use of nonlinear optical frequency-doubling; (3) The first GaN-based metal-semiconductor field-effect transistors (MESFET) were experimentally demonstrated in 1993 [13] and commercially available in 2010. High-speed field-effect transistors and high-temperature microelectronic devices were also developed using the material [2] in the 1990s; (4) GaN high-electron-mobility transistors (HEMTs) have been commercialized since 2006, applied at high efficiency and high voltage operation. GaN transistors can operate at much higher temperatures and work at much higher voltages than gallium arsenide transistors. GaN HEMTs are ideal power amplifiers at microwave frequencies; (5) GaN materials have also been utilized as renewable energy materials, such as in thermoelectric devices [14–17] to harvest waste heat, betavoltaic microbatteries [18–20] to collect energy from radioactive sources emitting beta particles, and solar cells [21] to collect solar energy. More applications of GaN materials can be found in some review literatures [1,2,4,22].

Micromachines 2016, 7, 121; doi:10.3390/mi7090121 www.mdpi.com/journal/micromachines Micromachines 2016, 7, 121 2 of 22

GaN is not sensitive to ionizing radiation, making it a suitable material working in radiation environments. Therefore, the GaN-based devices, including LEDs, LDs, MESFETs, HEMTs, and solar cells, have valuable applications in military and out space activities, showing stability in radiation environments. To date, GaN nanoparticles [23], nanorods [24–31], nanowires [32–41], and nanotubes [24,42–47] have been synthesized using a variety of techniques besides single crystals [48–52]. Compared with GaN crystalline bulks and films, GaN nanomaterials usually show tuning optical properties [53] and more interesting behaviors because of the quantum confinement, having wider applications [54] than bulks and films. Individual crystalline nanorods and nanowires have been utilized as nano-LEDs [55], LDs [56–58], and field effect transistors [59,60]. Assemblies of nanorods and nanowires have also been fabricated into nanowire LEDs [61–63], nanorod LEDs [64], and nano generators [65,66]. GaN nanoparticles have the highest surface / volume ratio among all GaN nanomaterials. Here we focus on free-standing self-assemblies of GaN nanoparticles. Syntheses and physical properties of GaN nanoparticles will be introduced, followed free-standing self-assemblies of GaN nanoparticles. The unique physical properties and potential applications of the free-standing assemblies are discussed at the end.

2. Syntheses of GaN Nanoparticles Hexagonal GaN nanoparticles have been synthesized by various techniques. Among these techniques, chemical vapor deposition, nitridation, solvothermal technique, and ball-milling are popular methods to produce GaN nanoparticles. Table 1 compares these four kinds of methods.

Table 1. Typical syntheses of gallium nitride (GaN) nanoparticles.

Method Precursors Catalysts Reaction Temperature (◦C) Average Diameter (nm) Reference CVD Ga - 900–1150 5–8 [67] nitridation Ga2O3 - 800–1100 40–500 [68] GaSb - 900 14–23 [69] Ga(NO3)3 - 850 5 [70] GaO(OH) - 900–1000 12–15 [71] solvothermal Ga † Li 350–500 30–70 [23] † Ga NH4I 350–500 12 [72] ‡ GaCl3 Li3N 280 30 [73] ball milling Ga and NH3 - 100 10–30 [74] †: ammono-thermal; ‡: benzene-thermal.

2.1. Chemical Vapor Deposition GaN nanoparticles can be easily prepared from a reaction of gallium and ammonia by chemical vapor deposition (CVD) methods at 900–1150 ◦C [5,49,67,68,75], and from organic gallium compounds by detonations or pyrolysis [76–78]. Figure1 shows spherical GaN particles synthesized from gallium by a CVD method. Gallium gas reacted directly with ammonia to form GaN nanoparticles. The diameters of the nanopaticles were about 5–8 nm, smaller than the exciton Bohr radius of about 10 nm. GaN nanoparticles were recently successfully synthesized via an microwave plasma-enhanced CVD method [79]. The resulting GaN nanoparticles had an average size of around 8.5 nm with a very narrow size distribution. GaN nanocrystals were also prepared from polymericgallium imide, (Ga(NH)3/2)n, and trioctylamine at 360 ◦C [80]. The produced GaN nanocrystals were spherical with a mean diameter of 30 ± 12 Å. The band gap of the GaN nanocrystals usually shifts slightly to higher energy because of the quantum conﬁnement. The detailed optical properties of GaN nanoparticles will be discussed in Section 3. 230 2 Results and discussion

A scanning electron microscopic image (Fig. 1a) shows that a layer of separated particles, which are distributed randomly, are deposited on the substrate surface. The particles have diameters of between 30 and 50 nm. We find that each particle is composed of agglomerated spherical particles that have diameters of about 5–8 nm (Fig. 1b) under larger magnification. The size is smaller than the exciton Bohr radius of about 10 nm. Electron dispersive analysis (EDS) shows that the particles contain only Ga and N. The X-ray powder diffraction pattern (Fig. 2) shows that the products are GaN with a wurtzite structure. Some diffraction peaks are broadened due to nano-sized effects. Transmission electron microscopy (TEM) observation revealed that no GaN wetting layer could be observed. The agglomerated dots seem to grow Fig. 2. X-ray powder diffraction pattern of GaN QDs with a wurtzite struc- directly on the LaAlO3 substrate surface. The combination of ture the NH3 flow rate, growth temperature, and growth time are found to be key factors in determining the dot size and the density of particles. In order to observe the substrate surface and the initial growth stage of QDs, we kept the substrate in the furnace at 820 ◦C for 10 s and immediately took it out. Figure 3 shows the scanning electron microscope (SEM) images of the substrate surface, The surface roughness can be clearly seen; meanwhile, the concave-surface-induced nucleation can also

Fig. 3. SEM observation of surface roughness and initial concave-surface- induced nucleation on a LaAlO3 substrate

Micromachines 2016, 7, 121 3 of 22 be observed. Facsko et al. [8] presented a formation process for semiconductor QDs based on a surface instability induced The CVD synthesized GaN nanoparticles usually crystallized well because of higher reaction by ion sputtering under normal incidence. They thought that temperature. However, there are nitrogen- or gallium-vacancies in CVD-grown GaN nanoparticles,the sputtering yield depended on the surface curvature, and affecting optical properties. The details will be discussed in Section 3. there was a competition between a roughening instability and It is challenging to produce massive GaN nanoparticles using the CVD methods becausediffusive of high smoothing mechanisms that governed the build-up cost of starting materials and low product yields. of a regular pattern with a characteristic wavelength. In our experiment, the surface diffusive smoothing process is greatly inhibited by the short growth time and low ﬂow rate of NH3 gas. The surface convexo-concave is a key factor for controlling the nucleation and growth of QDs. For the homogeneous and inhomogeneous nucleation process, the relationship between the formation energy of nucleation (∆G ∗) and the volume of critical crystal nuclei (V ∗) can be expressed as ∗ ∗ ∆G = V ∆GV /2, where ∆G V is the difference of free energy between two phases per volume unit. In order to obtain the same wetting angles, the critical volume of crystal nuclei on a convex surface (V ∗) is larger than that on an even smooth ∗ 1 ∗ surface (V2 ), and the volume on a concave surface (V3 )is the smallest under the identical degree of super-cooling (i.e. keeping the radius of critical crystal nuclei uniform). Hence, the relation between the three values of formation energy are Fig. 1a,b. GaN nanoparticles (a) which in turn are composed of smaller ∆ ∗ < ∆ ∗ < ∆ ∗ Figure 1. GaN nanoparticlesspherical synthesized nanoparticles by gas(b) synthesized reactions on by rough gas reactions LaAlO3 onsubstrates. a rough LaAlO Reprinted3 withG3 G2 G1 under the same conditions. As a re- permission from [67]. Copyrightsubstrate 2000 Springer. sult, the probability of inhomogeneous nucleation induced by

2.2. Nitridation Single-phase and high-pure GaN nanoparticles have been produced through nitridation of Ga-based materials. The Ga-based precursors are usually cheaper than gallium and GaN. It was reported that GaN nanoparticles were achieved via a reaction of gallium oxide with ammonia [68,81,82] over 1100 ◦C in ﬂowing ammonia. Pure GaN nanoparticles were synthesized through a nitridation reaction as expressed [68]:

Ga2O3(s) + 2NH3(g) → 2GaN(s) + 3H2O(g) (1)

The crystalline size of the produced GaN nanoparticles strongly depended on particle size of the Ga2O3 starting materials. Therefore, it is critical to employ nanoscale precursors to produce GaN nanoparticles. Additionally, the nitridation temperature varies with grain size of precursors. It was reported that the nitridation temperate was lowered to 800 ◦C when the grain size of gallium oxide nanoparticles was less than 20 nm [82]. Ga2O3 is much cheaper than Ga and the total cost of GaN nanoparticles would be reduced. Besides Ga2O3 powders, other gallium-based materials, such as gallium phosphide GaP microcrystalline particles [83], gallium antimonide GaSb powders [69], gallium arsenide GaAs powders [84], and gallium nitrate Ga(NO3)3 powders [70], were utilized to produce pure GaN nanoparticles under ammonia NH3 at high temperatures in one step nitridation procedures. The chemical reaction of the nitridation procedures was similar to Equation (1). A soluble salt nitration technique was recently developed [85] to synthesize GaN nanoparticles ◦ at low temperatures. Ga metal was mixed with N2SO4 powders and then heated at 700 C under NH3/N2 atmosphere. The weight ratio of Ga and Na2SO4 was 1:10, which was adequate to obtain ◦ ◦ high dispersion of Ga (melting point of 30 C) droplets onto Na2SO4 (melting point of 884 C) powders. GaN nanoparticles with mean diameter of 7.6 nm were produced in a large scale with a high yield (95.8%) through a direct nitridation reaction. Na3PO4 was also utilized as a dispersant [86] in the soluble salt-assisted technique. Large scaled high-crystalline GaN nanoparticles were produced ◦ through a direct nitridation of Ga-Na3PO4 mixture at 750–950 C. The mean diameter of the produced nanoparticles was 8–18 nm, depending on nitridation temperature. Micromachines 2016, 7, 121 4 of 22

Gallium oxide hydroxide was also employed as a precursor to produce GaN nanoparticles [71]. GaO(OH) nanoparticles were synthesized from a chemical co-precipitation method, and heated at high temperatures under ammonia. The average particle size of produced GaN powders was 12–15 nm when the nitridation temperature was 900–1000 ◦C. Nitridation is an economically effective method to synthesize massive GaN nanoparticles.

2.3. Solvothermal Techniques Solvothermal methods can synthesize various materials under supercritical conditions at high temperatures and high pressures [87]. Based on the used solvent type, the method is termed as hydrothermal (water as solvent), ammonothermal (ammonia as solvent), or benzene-thermal (benzene as solvent) method. Ammonothermal technique is an effective method to produce massive GaN nanoparticles from gallium at low temperatures [23,88]. Nanocrystalline GaN powders were synthesized under base [23,48] or acidic conditions [72,89] using various mineralizators. It was reported that gallium nitride nanoparticles were synthesized from gallium metal in stainless steel autoclaves at 350–500 ◦C in the presence of lithium mineralizator (pressure about 2000 atm at reaction temperatures) [23], potassium mineralizator [48], sodium mineralizator [48], and LiNH2 mineralizator [48]. The ammonia solutions were base because of the mineralizators. The average grain size of GaN powders was about 32 nm or less. When lithium was used as a mineralizator, alkali metal amide LiNH2 was produced in the solvent:

− + 2NH3 + 2Li → 2NH2 + H2 + 2Li (2) to form an ammono-basic solution. It was believed that intermediate compounds such as LiGa(NH2)4 were formed during the ammonothermal procedure and followed a reaction [23]:

LiGa(NH2)4 → GaN + LiNH2 + 2NH3 (3)

The chemical reaction was similar when potassium or sodium was employed as mineralizator. It was believed that alkali metal amide, Na[Ga(NH2)4][90] or K[Ga(NH2)4][91], was produced under the supercritial base-conditions. The intermediate compounds converted to GaN during ammonothermal procedures:

A[Ga(NH2)4] ↔ GaN + ANH2 + 2NH3 (4) where A is Na or K. Nanocrystalline GaN powders were also synthesized under acidic conditions [72,89]. Ammonium halides NH4X (X = F, Cl, Br, I) were typical ammonoacidic mineralizers to synthesize GaN in ammonothermal procedures. The ammonium halides increased concentration of ammonium ions + NH4 . Intermediate hexammoniates, such as [Ga(NH3)6]Br3 · NH3 and [Ga(NH3)6]I3 · NH3 [92], would form in ammonia and the produce Ga-containing ions mobiled in solutions. The Ga-containing ions decomposed to GaN nanoparticles at high temperatures [93]. GaN nanocrystaline particles can also be produced by a benzene-thermal synthetic route [73,94]. A benzene-thermal reaction of Li3N and GaCl3 was carried out [73] in which benzene was used as the solvent under high pressures at 280 ◦C. The size of the prepared GaN nanoparticles was 30 nm. The reaction temperature was much lower than that of traditional methods, and the yield of GaN reached 80%. The X-ray powder diffraction pattern indicated that the produced nanomaterials was mainly hexagonal-phase GaN with a small fraction of rocksalt-phase GaN. GaN nanoparticles were also synthesized at room temperature [95] when Li3N and GaCl3 was mixed with a ratio of 1:1 and kept in benzene and ether for hours. Ether greatly accelerated the reaction. Micromachines 2016, 7, 121 5 of 22

GaN spherical nanoparticles were also synthesized in superheated toluene [96]. Gallium chloride reacted with sodium azide in toluene to produce insoluble azide precursors. The produced precursors solvothermally decomposed to GaN at temperatures below 260 ◦C. The resulting GaN nanomaterials were poorly crystalline but thermally stable. Recently GaN nanoparticles were prepared in a tubular microreactor at supercritical conditions [97] using cyclohexane or hexane-ammonia solutions as solvents (temperature: ∼450 ◦C and reaction time: <25 s). The as-prepared nanoparticles exhibited a strong luminescence in the ultraviolet.

2.4. Ball-Milling Techniques Ball-milling is an effective industrial technique to produce massive GaN nanoparticles. Dependent on the type of ball-mills [22], various nanoparticles can be produced by mechanical grinding or mechanical alloying. GaN nanoparticles were also produced directly from GaN materials [28,98] by high-energy ball-milling through mechanical grinding. The particle size was reduced signiﬁcantly after ball-milling, conﬁrmed by X-ray diffraction and dynamic light scattering. Gallium nitride powders were synthesized by mechanical alloying of gallium metal in a dry ammonia atmosphere at elevated temperatures (about 100 ◦C)[74]. Transmission electron microscopy (TEM) indicated that the diameter of obtained GaN nanocrystals ranged between 10 and 30 nm. Larger crystals with a size around 70 nm were also observed. GaN nanoparticles were also synthesized by mechanochemical reaction between Ga2O3 and Li3N[99] through ball-milling under ammonia gas environment. The molar ratio of Ga2O3 and Li3N was 1:2. GaN nanopowders were prepared within 2 h with a mill speed of 300 rpm at room temperature.

2.5. Other Techniques Many other techniques have been developed to produce GaN nanoparticles. For examples, GaN nanoparticles were produced by pulsed laser ablation [100]. High quality GaN nanocrystalline were prepared by sol-gel method [101] with size of 30–100 nm. GaN nanoparticles were also produced from organometallic gallium azides at low temperature, such as 216 ◦C [102], in solutions. The crystalline quality of the produced materials was very poor while crystallized after annealing at 220 ◦C. Among these techniques, CVD method can produce high quality GaN nanoparticles while is cost. Ammonothermal technique, nitridation method, and ball-milling method can produce massive GaN nanoparticles economically.

3. Physical Properties and Applications of GaN Nanoparticles

3.1. Physical Properties of Crystalline Nanoparticles According to the classical thermodynamics, the energy band gap of GaN nanomaterials depends on crystalline size: α E = 1 + E (5) nano D bulk where the shape parameter α = 0.462 for spherical particles. The confinement-dependent exception binding energy of GaN nanoparticles was detected experimentally [103] from photoluminescence. The energy shifted to higher energy with decreasing particle size, in agreement with the theoretical prediction. The size-dependent band-gap was confirmed experimentally from photoluminescence of GaN nanoparticles. The photoluminescence spectrum of GaN nanoparticles usually blue-shifted [67,104,105] compared with that of GaN bulks (365 nm), which was ascribed to the quantum confinement effect. It was reported the band gap of the GaN nanocrystals shifted slightly to higher energy [80] once the diameter was about 30 nm. Its band edge emission with several emission peaks appeared in the range Micromachines 2016, 7, 121 6 of 22 between 3.2 and 3.8 eV at low temperature while there were two excited-state transitions at higher energies [80]. The melting temperature of GaN nanoparticles depends on the crystalline size too [106,107]. Numerical thermodynamical approach predicated the melting point of spherical GaN nanoparticles was 100–200 K lower than that of bulks. The size of GaN nanoparticles has great influence on dielectric behaviors of GaN nanoparticles [108]. Due to the existence of interfaces with a large volume fraction, hexagonal GaN nanoparticles have much higher dielectric constant than that of hexagonal GaN coarse-grain powders. The radiative decay time of excitons also depends on the GaN nanoparticle size [109]. Theoretical simulations indicated that GaN quantum dots had a long radiative decay time and was undesirable for optoelectronic applications as light-emitting diodes [110]. Therefore, GaN nanoparticles should be good candidates for the development of light emitting diodes, laser diodes, and novel optical devices in the short-wavelength region with a wide-wavelength tuning range [109].

3.2. Defects of GaN Nanoparticles There are four kinds of defects in GaN, zero-dimensional point defects, one-dimensional dislocations, two-dimensional planar defects, and three-dimensional volume defects. These defects affect optical, electrical, mechanical and thermal properties of gallium nitride, such as introducing strains in GaN materials, changing lattice constants and band-gap energies, forming donor or acceptor levels in band gaps. Up till now, few reports have discussed these defects of GaN nanoparticles. Compared with crystalline bulks and ﬁlms, GaN nanoparticles have a super high surface/volume ratio. The ratio is high up to 0.6 for a 10 nm diameter GaN crystalline spherical particle. The high surface/volume ratio would greatly reduce dislocation densities. When GaN nanoparticles are produced at high temperatures (such as 800–1000 ◦C in CVD and nitridation procedures), there are few three-dimensional defects produced during syntheses. Based on the facts of syntheses conditions and high surface/volume ratio, zero-dimensional defects and two-dimensional grain boundaries would dominate the defect effects of high-temperature grown GaN nanoparticles. Solvothermal grown GaN nanoparticles should have less defects, similar to ammonothermal grown GaN bulks [48,52]. CVD and nitridation grown GaN particles should produce Ga-vacancy defects or nitrogen-vacancy defects depending on nitrogen-rich or nitrogen-poor environments during high temperature procedures. Ball-milled nanoparticles should have more dislocations and planar defects because of mechanical collisions.

3.3. Defect Effects on Physical Properties Physical properties of nanoparticles are affected by various defects. Three kinds of defects (vacancies, surface defects, and grain boundaries) are mainly discussed below. Ga vacancies always exist in GaN nanoparticles. Figure2 shows a PL spectrum of GaN nanoparticles upon excitation at 325 nm recorded at room temperature. The diameter of the nanoparticles was 100–200 nm and the Ga/N ratio was 0.90. The spectrum exhibited three distinct features: a blue emission at 413 nm coming from the free excitonic transition between valence and conduction bands of stoichiometric GaN, a broad green luminescence (GL) centered at 467 nm, and a yellow luminescence (YL) band emission centered at 516 nm. The YL peak around 516 nm was saturated easily with excitation intensity, indicating the YL band should be attributed to Ga vacancy at surfaces [111]. The GL band at 467 nm showed almost no saturation, implying the band was related to isolated Ga vacancy (VGa). More detailed experiments showed that the optical absorption edge moved to longer wavelengths (red-shifted) as the concentration of Ga vacancies increased [112]. Figure3 shows the effect of Ga vacancy on UV-vis spectrum of GaN nanoparticles. The band-gap of GaN nanoparticles increased with decreasing concentration of Ga vacancy. The band-gap of Ga0.90N nanparticles with the concentration Micromachines 2016, 7, 121 7 of 22

of Ga vacancy of 2.8% was 3.15 eV, 3.17 eV for Ga0.95N nanoparticles with the concentration of Ga vacancy of 1.8%, and 3.20 eV for Ga0.98N nanoparticles with the concentration of Ga vacancy of 1.6% [112].

Ga0.90N Ga0.95N Ga0.98N

Nitrogen vacancies were observed in some GaN nanoparticles synthesized under nitrogen-absence environments [113]. The dielectric properties of these N-deﬁcient GaN nanoparticles exhibited signiﬁcant enhancement over that of GaN nanomaterials at low frequency range. Doping can induce various defects in GaN nanoparticles and affect optical, magnetic properties. Here we do not discuss it. Surface defects also affect photoluminescence [32,114]. Similar to Ga vacancies or nitrogen vacancies just discussed, the surface defects changed band gaps and shifted optical absorption edges of GaN nanoparticles. Surface defects also affect magnetic properties of GaN nanomaterials. GaN bulks are diamagnetic. However, un-doped GaN nanoparticles showed ferromagnetic at room temperature [115] when the average diameter of the nanoparticles was in the range 10–25 nm. Furthermore, the saturation magnetic moment decreased with increasing nanoparticles size, suggesting that ferromagnetism was due to the surface defects of the nanoparticles. Recently experiments indicated the ferromagnetism should be induced by Ga vacancies [112]. Micromachines 2016, 7, 121 8 of 22

Among two-dimensional defects of GaN nanoparticles, grain boundaries play a more important role than stacking faults, twins and inversion domain boundaries. The effects of grain boundaries can be described as the size effect and surface defects, as discussed above. Their effects on optical properties can be described by Equation (5). Dislocations in GaN materials have played an important role of optical properties, especially for GaN ﬁlms and GaN bulks. Here the effect is not discussed because the one-dimensional defects do not dominate physical properties of GaN nanoparticles. Three-dimensional defects, such as voids, cracks and nanopipes are usually observed in bulks and ﬁlms. However, this kind of defects is rarely observed in nanoparticles and not discussed here. More detailed information of luminescence properties of defects in GaN can be found in previous review literatures [116].

3.4. Applications of Nanoparticles GaN nanoparticles have been deposited on substrates as solar cells [117]. The conversion efficiency is 3.10% under air mass 1.5 global illumination and room temperature conditions. A further increase of 15% was achieved in short circuit current density, improving the conversion efficiency to 3.87%, in an optimized structure. GaN nanoparticles with size of 15–20 nm were added to poly(3-hexylthiophene) (P3HT) as hybrid solar cells [118]. With the addition of GaN nanoparticles to P3HT from 5 to 15 mg/mL, the performance of the hybrid solar cells was greatly enhanced. The short circuit current density was tripled to 3.5 mA/cm2 and filling factor to 44% from 20%. GaN nanoparticles were employed as photocatalysts for overall water splitting [119]. The photocatalytic activity of GaN for the reaction was found to be strongly dependent on the crystallinity of GaN nanoparitcles. Compared to bulk materials, GaN nanoparticles have larger surface area, size-dependent properties, increased absorption coefficient, increased band-gap energy, and reduced carrier-scattering rate, offering potential advantages than bulks and films.

4. Free-standing Self-assembly of GaN Nanoparticles GaN nanoparticles were self-assembled into various 2D macroscale structures on substrates, working as LEDs and LDs. 3D free-standing structures, such as porous nanotube arrays [120] and crystalline nanotube arrays [43,121] were also fabricated by templates. Here we focus on free-standing GaN porous materials grown without any templates.

4.1. GaN Nanospheres Figure4 shows a GaN nanosphere [ 122] with a diameter of 20–25 nm. The nanosphere was self-assembled from GaN nanoparticles with a diameter of several nanometers. It was believed that there were lots of gallium nanodroplets around gallium sources. When ammonia NH3 gas was introduced into reaction chambers, NH3 molecules were easily adsorbed onto surfaces of the nanosized gallium droplets. Then GaN nanoparticles nucleated at the surfaces of the metallic gallium nanodroplets. With increasing reaction time, more and more NH3 passed though the previously formed GaN-Ga interfaces to react with Ga to form GaN shells. Hollow spheres were formed when the gallium nanodroplets were consumed. The produced hollow spheres were composed of polycrystalline nanoparticles. Sometimes the hollow GaN spheres further aggregated into columns, as shown in Figure5. Semiconductor GaN Hollow Spheres and Nanotubes ing hollow GaN nanospheres by carefully controlling the treatment temperature and the ammonia gas partial pressure. In the synthesis process, the nanosized Ga liquid droplets are first deposited on a silicon or sapphire substrate using GaCl3 as gallium precursor, and the hollow GaN spheres are subsequently grown by an in situ chemical reaction between the liquid Ga formed and introduced NH3 in which the gallium droplets do not act as a catalyst, as reported previously,[25] but rather act as a reactant and a template for the formation of hollow GaN structures. Typically, the diameter of the synthesized hollow GaN spheres and nanotubes is 20–25 nm and the shell thicknessMicromachines of the2016, hollow7, 121 9 of 22 GaN nanostructures is 3.5–4.5 nm. The room-temperature photoluminescence spectra for the synthesized GaN spheres and nanotubes display a strong, blue-shifted band-edge emission peak at 3.52 eV (352 nm).

2. Results and Discussion

After the synthesis a yellow bulk product collected on the silicon substrate where the zone temperature was measured to be 6508C. The as-prepared products were charac- terized by X-ray diffraction. Figure 1a shows a typical XRD Figure 4. HRTEM image of aFigure GaN nanosphere. 2. a) Low- Reprinted and b) with high-magnification permission from [122]. TEMimages Copyright 2005 of the syn- WILEY-VCH Verlag GmbH & Co. Semiconductor GaN Hollow Spheres and Nanotubes thesized hollow spherical particles. c) A magnified TEMimage of a single hollow spherical particle. d) Selected-area electron diffraction tion of hollow GaN spheres. This process is similar to that (ED) pattern from a synthesized hollow sphere suggesting the syn- of InP quantum nanostructure growth by the in situ reaction [27] thesized hollow sphere is composed of polycrystalline GaN with a of indium droplets with phosphide ions. When the NH3 gas is introduced, because the surface area of the nanocrys- wurtzite structure. tals is much larger than that of the bulk materials, and there are a lot of vacancies and dangling bonds on the surface of the Ga liquid droplets, the NH3 molecules are easily adsorbed onto the surface of the starting nanosized Ga liquid sized GaN particles. Figure 2c depicts a high-magnification droplets. The surface of the metallic gallium liquid droplet TEM image of a typical single hollow sphere particle. A is the location where GaN nucleates. NH3 reacts with gallium to form GaN on the surface of the Ga droplets. With in- contrast difference between the shell section and center of creasing time, more and more NH3 passes though the previ- the particle can be observed, clearly indicating the hollow ously formed GaN–liquid Ga interface to react with Ga and form GaN to finally give hollow spheres composed of small, structural nature of the synthesized product. Figure 2d is a polycrystalline particles. The formation process of hollow typical selected-area electron diffraction (ED) pattern from GaN spherical nanocrystals is shown schematically in Fig- ures 5a–c. As the Ga droplet serves as a template and a re- an as-synthesized hollow sphere. The diffraction rings in the actant it strictly limits the lateral growth of individual ED pattern correspond well with that of wurtzite-type GaN, hollow GaN spherical nanocrystals. The formation of hollow Figure 1. XRD spectraGaN of nanotubes a) GaN at hollow higher temperature spheres, and is easy b) to GaN understand hollow directly indicating that the spherical particle is composed of nanotubes. as at these temperatures the formed hollowFigure GaN 5. spheresTEM imageFigure a GaN 6.polycrystallineA sphere TEMimage column. of the aligned Reprinted GaN hollow with GaN with permission spheres; a wurtzite hollow from [122]. structure. Copyright 2005 Typically, the aggregate, and the hollow GaN nanotubes could be formed GaN nanotubes could be formed by combination of these hollow WILEY-VCH Verlag GmbH & Co. by the coalescence of the nanosized hollow GaN spheres, as GaN spheres.synthesized hollow spheres are relatively uniform in size, illustrated schematically in Figures 5d and e. The formation ranging from 20 to 25 nm in diameter. process of hollow GaN nanospheres is4.2. very GaN similar Nanotubes to that [28] [29] spectrum fromof the ZnO synthesized shells reported by product. Wang et al. TheGaNFigure porous 6reflection shows nanotubesby Shi were et al.A self-assembledBoth high-resolution the shell from thickness GaN nanoparticles transmission of the hollow [122]. electron Figure6a shows microscope some aligned hollow GaN spheres with diameters of 20– spheres and the wall thickness of the nanotubes are in the peaks could be indexed to wurtzite GaN withseveral lattice GaN nanotubes. param- The darker(HRTEM) edges indicated attached that tothe an GaN X-ray materials energy were hollow dispersive tubes, not spectrom- 25 nm, and the hollow GaN polycrystalline nanotubes could range 3–5 nm, and the synthesized products have a uniform eters of a=3.186be formed and byc the=5.178 combination (JCPDS: of suchsolid hollow rods. 02-1078). spheres, The wall the thicknesses Nosize distribution.eter were 3.5–5.0(EDS)nm. were The GaN used nanotubes to further had a length characterize of several hundreds the synthe- other impuritiesdiameter could of the be hollow indexed GaN nanotubes fromof basically nanometers. the synthesized depending SelectedFigure areasized electron 7 depicts hollow diffraction a typical spheres room-temperature (SAED) both patterns microstructurally PL (inset spec- of Figure6b) indicated and chemically. on that of the hollow GaN spheres. Thethat process the nanotubes for the for- weretrum polycrystalline. for the as-synthesized Figure6 hollowb shows GaN detailed spheres, nanostructures and almost of the nanotubes. products. mation of GaN nanotubes from hollowThe spheres nanotube is similar walls to werethe composed sameFigures PL spectrum of single-layer-ed 3a is and obtained b show GaN for the nanocrystalline two synthesized HRTEM GaN particles images with a for diameter the shells of The synthesizedthat of products Co3O4 nanotubes were from sonicated colloidalof 3.0–3.5 particles in ethanol nm. reported andnanotubes.two Bulk different GaN shows hollow a band-edge spheres. emission The at shell section and core sec- 3.40 eV (365 nm). The PL spectrum of the synthesized It was believed that the hollow GaN nanotubes were formed by the coalescence of the nanosized dispersed onto copper grids coated with an amorphoushollow GaNtion spheres of the displays synthesized a sharp emission hollow peak, namely sphere can clearly be seen, hollow GaN columns shownan ultraviolet in Figure (UV)5. near-band-edge transition, at 3.52 eV carbon film to carry out a transmission electron microscopy(352 nm).and The also observed that strong the shell band-edge of the emission hollow at spheres is very thin and (TEM) examination of the morphologies and structures of3.52 eVcomposed (352 nm) is blue-shifted of very relative small to nanoparticles. that of bulk The thickness of the GaN by about 120 meV (13 nm). This should be attributed the resulted products. Figure 2a and b show low-magnifica-to a quantumshell confinement is in the effect,range because 3.5–4.5 the synthesized nm. The inset in Figure 3a is the tion TEM images of the synthesized hollow spherical parti- EDS spectrum obtained from the synthesized hollow cles. They display pale regions in the central parts that con- sphere; it shows that the hollow sphere is composed of Ga trast with the dark shells or edges. When the sample was and N, with a chemical composition of 1:0.99 (Ga:N ratio) continuously tilted, the light contrast in the central part did corresponding to the expected stoichiometry of bulk GaN. not vary in contrast and size. This excludes the possibility of The Cu signals come from the copper grid. It should be solid spheres and suggests the hollow nature of the synthe- noted that several tens of hollow GaN spheres were charac- small 2005, 1, No. 11, 1094 – 1099 2005 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim www.small-journal.com 1095

full papers L.-W. Yin et al. Micromachines 2016, 7, 121 10 of 22

40 nm (a) (b) Figure 4. a) Low- and b) high-magnification TEMimages of the hollow ◦ GaN nanotubes synthesized at a temperature above 1100 C. c) An FigureFigure 3. a,b) High-resolution 6. (a) Typical transmission TEM electron image microscopy of GaN nanotubes synthesized above 1100 C by a gas8 interface (HRTEM) images of the shell of two hollow GaN spheres. The inset in HRTEMimage taken from the wall of a single nanotube. The inset is reaction route; (b) Cross-sectional HRTEM imagea SAED of a pattern nanotube. of a single The nanotube, inset suggesting is an SAED that the pattern synthesized of the (a) is the EDS spectrum of the shell of a hollow GaN sphere. nanotube. Reprinted with permission from [122].nanotube Copyright is polycrystalline. 2005 Wiley-VCH Verlag GmbH & Co. terized by EDS and the same results were obtained for all 4.3.of them. GaN The Circular synthesized Microtubes hollow GaN sphere is very stable The formation of the hollow GaN spherical nanostruc- even under strong electron beam irradiation. To the best of tures is proposed to be a process composed of the following our knowledge,Figure7 suchshows hollow wurtzite GaN spheres GaN with microtubes a shell thick- grownstages: 1)generation by a CVD of small, method nanosized without liquid Ga templates droplets [ 123]. ness of 3.5–4.5 nm have never been reported. on the substrate, 2) surface nitridation of Ga droplets, and

GalliumThe products was placed obtained on could quartz be controlled substrates by carefully in a hot-walled3) a continuous CVD in system.situ liquid Thengallium–NH Ga3 reactedgas interface with re- ammonia atadjusting 820–840 the◦ synthesisC and temperature yellow GaN at a given nanostructures ammonia par- action. were Based produced on the starting on the materials, quartz the substrates. yields of GaN Scanning tial pressure. When we increased the treatment temperature, hollow nanospheres and nanotubes obtained at different electronhollow nanotubes microscopy could be (SEM) synthesized indicated on the silicon that sub- thesynthesis yellow temperatures deposits are were about microtubes 37% and 32%, respective-(Figure7a). The diameterstrate where of the the temperature hollow microtubes was measured to was be 700about8C. 8ly.µ Itm shouldand belengths noted that up during to 100 theµ firstm. stage, The the ends initial of most of The XRD pattern for the synthesized GaN nanotubes is deposition and formation of nanosized Ga liquid particles thesegiven microtubes in Figure 1b, suggesting were open, the product but some is wurtzite-type were closedor droplets (Figure on7 theb–d). Si or sapphire substrate is of great impor-

GaN. Figures 4a and b show low-magnification TEM tance for the final formation of hollow GaN spheres. GaCl3 images of the hollow nanotubes obtained. The contrast dif- has a melting point of 351 K and a boiling point of 473 K.

ference between the pore(a) section and the wall section can Under(b) thermal heating, the GaCl3 is vaporized and transfer- clearly be seen, suggesting the tubular nature of the synthe- red by the Ar gas to the low-temperature¬ zone. The Ga sized products. The synthesized nanotubes are typically 20– formed by decomposition of the GaCl3 deposits on the sili- 25 nm in outer diameter and the wall thickness of the tubes con substrate.¬ Ga has a low melting point of 302.8 K and a is 3.5–5.0 nm. Figure 4d is a SAED pattern obtained from a boiling point of 2520 K, therefore large amounts of Ga single nanotube, suggesting that the synthesized nanotube is liquid droplets form on the silicon or sapphire substrate. polycrystalline with a wurtzite-type GaN structure. From This is also supported¬ by the Sacilotti groups recent report Figure 4. a) Low- and b)the high-magnification HRTEM image obtained TEMimages from the wall of of the a single hollow GaN of three-dimensional (3D) Ga balloon¬ formation on a sili- GaN nanotubes synthesized at a temperature above 1100 C. c) An 30 µm [26] 10 µm Figure 3. a,b) High-resolution transmission electron microscopy nanotube (Figure 4c), it can clearly be seen that8 the nano- con substrate. During the subsequent GaN formation tube wall is composed of small nanocrystals with a diameter stage, the Ga liquid droplets or balloons do not act as a cat- (HRTEM) images of the shell of two hollow GaN spheres. The inset in HRTEMimage taken from the wall of a single nanotube. The inset is a SAED pattern of a singleof 3–3.5 nanotube, nm. suggesting(c) that the synthesized alyst(d) but rather as a reactant and a template for the forma- (a) is the EDS spectrum of the shell of a hollow GaN sphere. nanotube is polycrystalline.1096 www.small-journal.com 2005 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim small 2005, 1, No. 11, 1094 – 1099 terized by EDS and the same results were obtained for all of them. The synthesized hollow GaN sphere is very stable The formation of the hollow GaN spherical nanostruc-¬ ¬ even under strong electron beam irradiation. To the best of tures is proposed to be a process composed¬ of the following 10 µm 10 µm our knowledge, such hollow GaN spheres with a shell thick- stages: 1) generation of small, nanosized liquid Ga droplets 1 2 ness of 3.5–4.5 nm have never been reported. on the substrate, 2) surface nitridation(e) of Ga droplets, and (f) 1 0 The products obtained could be controlled by carefully 3) a continuous in situ liquid gallium–NH3 gas interface re- adjusting the synthesis temperature at a given ammonia par- action. Based on the starting materials, the yields of GaN 8 s t 6 n

¬ u o tial pressure. When we increased the treatment temperature, hollow nanospheres and nanotubes obtained at different C 4 hollow nanotubes could be synthesized on the silicon sub- synthesis temperatures are about 37% and 32%, respective-¬ 2 strate where the temperature was measured to be 7008C. ly. It should be noted that during the first¬ stage, the initial 1 µm 0 0 5 1 0 1 5 2 0 The XRD pattern for the synthesized GaN nanotubes is deposition and formation of nanosized Ga liquid particles D i a m e t e r ( m i c r o n ) given in Figure 1b, suggesting the product is wurtzite-type or droplets on the Si or sapphire substrate is of great impor- Figure 7.FigureSEM 2: images SEM images of (a of–d (a-d)) GaN microtubules microtubes grown grown at diﬀerent on quartz regions substrates and (e) wall and surface (e) surfaceof of GaN. Figures 4a and b show low-magnification TEM tance for the final formation of hollow GaN spheres. GaCl3 a microtubule. White arrows mark the closed ends, black arrows the open ends, and the blue images of the hollow nanotubes obtained. The contrast dif- has a melting point ofa 351 microtube. K and White a boiling arrows point mark of the 473 closed K. ends, black arrows the open ends, and blue arrows arrows the nanopores of the microtubules. (f) Histogram of microtubule diameter. ference between the pore section and the wall section can Under thermal heating,the the nanopores GaCl is of vaporizedthe microtubes; and (f transfer-) Histogram of microtube diameter. Reprinted with permission from [123]. Copyright3 2015 Elsevier. clearly be seen, suggesting the tubular nature of the synthe- red by the Ar gas to the low-temperature zone. The Ga sized products. The synthesized nanotubes are typically 20– formed by decomposition of the GaCl3 deposits on the sili- 25 nm in outer diameter and the wall thickness of the tubes con substrate. Ga has a low melting point of 302.8 K and a is 3.5–5.0 nm. Figure 4d is a SAED pattern obtained from a boiling point of 2520 K, therefore large amounts of Ga single nanotube, suggesting that the synthesized nanotube is liquid droplets form on the silicon or sapphire substrate. polycrystalline with a wurtzite-type GaN structure. From This is also supported by the Sacilotti groups recent report the HRTEM image obtained from the wall of a single GaN of three-dimensional (3D) Ga balloon formation on a sili- [26] nanotube (Figure 4c), it can clearly be seen that the nano- con substrate. During the subsequent GaN formation 5 tube wall is composed of small nanocrystals with a diameter stage, the Ga liquid droplets or balloons do not act as a cat- of 3–3.5 nm. alyst but rather as a reactant and a template for the forma-

1096 www.small-journal.com 2005 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim small 2005, 1, No. 11, 1094 – 1099 Micromachines 2016, 7, 121 11 of 22

The thicknesses of the microtube walls were roughly measured from the open ends of the microtubes, about 100 nm. Detailed SEM examination (Figure7e) indicated the walls were consisted of single layer of randomly oriented nanostructures. The diameters of the nanostructures were about 100 nm and their lengths were about 1 µm. SEM images (Figure7e) also indicated that the thin walls of the microtubes were porous because of the random packing of the nanostructures. Some nanopores of these walls are notated by blue arrows in Figure7e. Such porous microtubes should have large surface areas. X-ray powder diffraction indicated that the microtubes were hexagonal GaN nanomaterials [123]. The phase was also conﬁrmed by SAED on TEM.

4.4. GaN Squared Microtubes Squared GaN microtube were also synthesized by the CVD method [124], as shown in Figure8a. The microtubes were several microns long. The thicknesses of the microtube walls were about 100 nm. More detailed SEM images (Figure8b) indicated that the walls consisted of single layer of random nanoparticles. (a) (b)

3 um

Figure9 shows a selected area electron diffraction (SAED) pattern taken from a nanoparticle of the squared microtubes. The nanoparticle was single crystalline. The SAED pattern had a three-fold symmetry and was indexed as the cubic GaN phase with the space group of F4¯3m along a zone axis of [111] (the lattice parameter a = 4.503 Å). Therefore, the squared microtubes should be consisted zinc-blende GaN nanoparticles.

[111]

The formation of the porous microtubules can be explained by the Liesegang phenominon and diffusion limited growth. Large amounts of gallium liquid droplets condense on quartz substrates from gaseous gallium. Gallium nanodroplets evaporate at high temperatures to produce gallium vapor. Once ammonia passes the gallium nanodroplets, gallium vapor would react with ammonia to produce GaN nanoparticles (nanorods or irregular nanostructures). The number of the GaN nanoparticles per unit volume is so dense around the gallium droplets that the GaN nanoparticles can be considered as a supersaturated aerosol around the gallium droplets. These supersaturated GaN nanoparticles would spontaneously aggregate into Liesegang rings because of thermodynamatic stability [125,126]. GaN nanoparticle Liesegang ringsX.L. were Chen observed et al. / Journal of [ Crystal124] Growth and 209support (2000) 208 the}212 hypothesis. 211 Then porous GaN microtubules grow on the Liesegang rings. Based on the diffusion-limited aggregation mechanism, the protruding parts of the GaN microrings would easily attract the GaN nanoparticles and grow quickly to build a new nanoparticle-wall from the Liesegang ring edges, leaving the hollow interior. With the consuming of gallium, supersaturated GaN nanoparticles would be continuously synthesized from the reaction of ammonia and gallium vapor as long as the gallium vapor pressure are high enough, resulting in the continuous deposition of nanoparticles on the GaN microrings to form GaN microtubules. During the whole growth stage, the gallium droplets would evaporate to generate the gallium vapor to keep the microtubules open or the microtubes would be closed if the gallium nano droplets were consumed before the experiments ended.

4.5. GaN Nanocomposite Bulks GaN nanocomposite bulks were in-situ fabricated under ammonothermal conditions. The GaN nanocomposite was directly produced through reactions of Ga metal and ammonia with NH4Cl catalysts in autoclaves [72]. Figure 10a shows a typical photograph of a fragment of the nanocomposites. The size of the fragment was about 0.7 × 0.6 cm2 with an thickness of about 0.2 cm. The fragment was transparent to visible light. SEM examinations revealed the existence of terrace steps which resemble characteristics of brittle materials. The assemblies were bulk materials like polycrystalline ceramic materials, not simple mechanical aggregation of grains.

(a) (b)

210 X.L. Chen et al. / Journal of Crystal Growth 209 (2000) 208}212

Fig. 3. (a) TEM image of morphology(c) of the nanosized grains in NAB GaN. (b) TEM powder electron di!raction patterns. (c) TEM nano area di!raction pattern on a single nanosized grain. (d) HRTEM image taken along [0 0 1] axis. (e) HRTEM image taken along [1 0 0] axis. (f ) HRTEM image showing a pore resulting from the packing of three grains of [0 0 1] orientation. The images were recorded on a Hitachi H-9000 HRTEM.

Fig. 2. Powder X-ray di!raction pattern for the nanocrystal-assembled bulk GaN. Figure 10. (a) Optical photograph of a fragment of GaN nanocomposites. The grid spacings 2 (Fig. 3c) on a single grain taken along the [0 0 1] sionally, opaque samples were obtained where are 0.2 × 0.2 cm ;(axisb) gives HRTEM further evidence image that of the three nanosized adjacentmuch larger nanograins; pores were indeed observed. (c) An XRD pattern of GaN grains are hexagonal GaN. Figs. 3d and e show The formation mechanism of NAB and nano- nanocomposites. Reprintedtypical high-resolution with permission transmission electron from [72crystalline]. Copyright GaN is not clear. 2000 A similar Elsevier. method was microscopy (HRTEM) images taken along [0 0 1] recently reported to produce GaN microcrystals and [1 0 0] zone axes of two grains of 18 and 12 nm with lithium, potassium compounds or rare earth in size, respectively. It can be seen that the elements as mineralizers [15]. In the present pro- nanosized grains in the NAB GaN are well crystal- cess, the addition of NHCl is indispensable in lized. The lattice constants calculated from the forming GaN at such a low temperature &3503C. fringe spacings of (1 0 0) and (0 0 1) planes are A possible mechanism is proposed as follows. " s " s a 3.1 A and c 5.2 A, in good agreement with NHCl is known to be very soluble in liquid am- the above values determined by powder X-ray dif- monia (124 g/100 g NH) and the resulting solution fraction pattern. The morphology of the atom clus- possesses a strong acidity [16]. Metal Ga may ters observed from HRTEM images along di!erent be easily oxidised to produce Ga> ions and axes indicates that the clusters are rather equiaxed. form soluble intermediate compounds such as " The NAB GaN also contains pores as in the Ga(NH)LCl(n 1}14) [16] in such a solution. nanophase materials formed by sintering or com- Then nano-sized GaN will precipitate from either pacting nanosized powders. Fig. 3f shows HRTEM the decomposition or further reactions of these images of a pore resulting from the packing of three intermediate compounds with liquid ammonia. grains with [0 0 1] orientation. The size of these Since there is a strong aggregating tendency for pores is estimated to be around &5 nm, roughly nanosized clusters in the solution like the case with half of the average diameter of grains. The density small clusters in a colloid, these nanocrystals are for NAB GaN measured by Archimedes den- consolidated into NAB GaN when the pressure in sitometry is 72% of that for the GaN single crys- an autoclave is high enough. Consolidation of ox- tals. This value approaches the theoretical limit for ide "ne particles was already known to be e!ective close packing of identical spheres. Since the size of in hydrothermal process [13]. pores is well below the wavelengths of visible light, In conclusion, nanocrystal-assembled bulk GaN the NAB GaN is transparent to visible light. Occa- was synthesized by a one-step method. The bulk Micromachines 2016, 7, 121 13 of 22

The GaN nanocomposite bulks were consisted of nanograins with several nanometers. Figure 10b shows an HRTEM image of three adjacent GaN nanograins with [001] orientation. Each adjacent nanograin was chemically bounded with others. Among the three adjacent nanograins, a nanopore was generated. The size of these pores was around 5 nm, roughly half of the average diameter of grains. Since the size of pores was well below the wavelengths of visible light, the GaN nanocomposites was transparent to visible light as shown in Figure 10a. The exist of nanopores in the nanocomposites was indirectly conﬁrmed by mass density measurements. The measured mass density for the GaN nanocomposites was 72% of that for the GaN single crystals. This value approached the theoretical limit of close-packing of equal spheres. Figure 10c shows the XRD pattern of the GaN nanocomposites. The XRD pattern indicated that the assembly was hexagonal GaN. All reﬂections were broadened due to the size effect. The average grain size was 11.8 nm according to the Scherrer formula, in agreement with HRTEM observation in Figure 10b. The formation mechanism of GaN nanocomposites is not clear. A possible mechanism was proposed as follows [72]: (1) NH4Cl is very soluble in liquid ammonia (124 g/100 g NH3) and the resulting solution possesses a strong acidity; (2) Metal gallium reacts with ammonia to produce Ga3+ ions and form soluble intermediate compounds such as Ga(NH3)nCl3 (n = 1 − 14)[127] in such a acidic solution; (3) Then nano-sized GaN particles would precipitate from either the decomposition or further reactions of these intermediate compounds with liquid ammonia; (4) There was a strong aggregating tendency for nanosized clusters in the solution and the GaN nanocrystals were consolidated into GaN nanocomposites when the pressure in an autoclave is high enough.

5. Optical Properties of GaN Self-assemblies The GaN self-assemblies talked here are consisted of nanoparticles whose optical properties strongly depend on crystalline size. Their photoluminescence and Raman scattering are discussed in this section.

5.1. Photoluminescence Figure 11a shows a typical photoluminescence (PL) spectrum of an individual GaN circular microtube (Figure7), excited by 532 nm wavelength radiation. The laser beam was focused on the individual microtube under an optical microscope to excite the PL spectrum. A yellow band, centered at 653 nm, was observed. Similar yellow bands were also observed in nanoparticles [23] and nanowires [32] with wurtzite structure.Y. Lan et al. / Journal of Crystal Growth 415 (2015) 139–145 143

20 20

15 15

10 10 Intensity (a.u.) Intensity (a.u.)

5 5

0 0 550 600 650 700 750 800 550 600 650 700 750 800 Wavelength (nm) Wavelength (nm)

Fig. 5. Photoluminescence spectra of (a) a GaN circular microtube with wurtzite structure and (b) a GaN squared microtube with zinc-blende structure. Figure 11. Photoluminescence spectra of (a) a GaN circular microtube with wurtzite structure and (b) a GaN squared microtube with zinc-blende structure. Reprinted with permission from [123]. Copyright 2015 Elsevier.

Ga Ga quartz quartz quartz quartz

: substrate

: GaN nanorod

: Ga vapor

: Ga droplet : Liesegang ring of GaN

Initial Stage Template Stage Growth Stage Enclosing Stage

Fig. 6. Possible growth mechanism of the self-assembled GaN microtubules. (a) Initial stage. No GaN nanomaterials are synthesized. (b) Template stage. Liesegang rings of GaN nanomaterials are formed. (c) Growth stage. GaN nanorods self-assemble into microtubules. (d) Enclosing stage. Ga sources are exhausted and the ends of GaN microtubules are closed. Top: model of each stage. Middle and bottom: SEM images at each stage. the consuming of gallium, supersaturated GaN nanorods are con- Additionally, the GaN microtubules are synthesized in a short time tinuously produced from the reaction of ammonia and gallium vapor – only 2 min. It is the indirect evidence for the supersaturation as long as the gallium vapor pressure are high enough, resulting in growth, indicating rapidity of the Liesegang ring formation and the the continuous deposition of GaN nanorods on the GaN microrings microtubule growth. These two phenomena (ring formation and to form GaN microtubules. During the whole growth stage, the rapid growth) are the specific characteristics of super-saturation gallium droplets evaporate to generate the gallium vapor to keep the growth caused by the Liesegang effect. microtubules open. The gallium droplet size is reduced with the reaction time because of evaporation. Once the gallium droplets vanish, the vapor 4. Conclusion pressure of the gallium droplets would decrease near the open ends of GaN tubules. As a result, the gallium vapor pressure front should One kind of novel gallium nitride circular tubules is grown from shrink back from the microtubule open tips. Because the GaN wurtzite nanorods by chemical vapor deposition. The bottom-up nanorods aggregate along the gallium vapor pressure front, the grown microtubules are porous, several tens of micrometers long and aggregating GaN nanorod front would also shrink back to enclose several microns in diameter. The constituent nanorods are crystalline the microtubule ends, forming enclosed GaN microtubes as shown in with lots of defects. The self-assembly of GaN nanomaterials is Fig. 6d. The SEM images in Fig. 6d show two microtubules with half- contributed to the diffusion limited growth of supersaturated GaN closed ends. nanorods. Compared with bulk crystals, the Raman active modes of Semiconductor GaN Hollow Spheres and Nanotubes tion of hollow GaN spheres. This process is similar to that of InP quantum nanostructure growth by the in situ reaction [27] of indium droplets with phosphide ions. When the NH3 gas is introduced, because the surface area of the nanocrystals is much larger than that of the bulk materials, and there are a lot of vacancies and dangling bonds on the surface of the Ga liquid droplets, the NH3 molecules are easily adsorbed onto the surface of the starting nanosized Ga liquid droplets. The surface of the metallic gallium liquid droplet is the location where GaN nucleates. NH3 reacts with gallium to form GaN on the surface of the Ga droplets. With increasing time, more and more NH3 passes though the previously formed GaN–liquid Ga interface to react with Ga and form GaN to finally give hollow spheres composed of small, polycrystalline particles. The formation process of hollow GaN spherical nanocrystals is shown schematically in Fig- ures 5a–c. As the Ga droplet serves as a template and a reactant it strictly limits the lateral growth of individual hollow GaN spherical nanocrystals. The formation of hollow GaN nanotubes at higher temperature is easy to understand as at these temperatures the formed hollow GaN spheres Figure 6. A TEMimage of the aligned hollow GaN spheres; hollow aggregate, and the hollow GaN nanotubes could be formed GaN nanotubes could be formed by combination of these hollow by the coalescence of the nanosized hollow GaN spheres, as GaN spheres. illustrated schematically in Figures 5d and e. The formation process of hollow GaN nanospheres is very similar to that [28] [29] of ZnO shells reported by WangMicromachines et al. Figure2016 6, shows7, 121 by Shi et al. Both the shell thickness of the hollow 14 of 22 some aligned hollow GaN spheres with diameters of 20– spheres and the wall thickness of the nanotubes are in the 25 nm, and the hollow GaN polycrystalline nanotubes could range 3–5 nm, and the synthesized products have a uniform be formed by the combination of such hollowFigure spheres, 11b shows the asize PL spectrumdistribution. of the square microtubes shown in Figure8. A strong and wide diameter of the hollow GaN nanotubes basically depending Figure 7 depicts a typical room-temperature PL spec- yellow band was observed at 550–750 nm. It is generally accepted that nitrogen vacancies and deep on that of the hollow GaN spheres. The process for the for- trum for the as-synthesized hollow GaN spheres, and almost mation of GaN nanotubes from hollowlevel spheres impurities is similar contribute to the to same the PL yellow spectrum band. is obtained for the synthesized GaN that of Co3O4 nanotubes from colloidalFigure particles 12 reported shows a typicalnanotubes. room-temperature Bulk GaN shows PL spectrum a band-edge of GaN emission nanospheres at shown in Figure4. The PL spectrum of GaN3.40 hollow eV (365 spheres nm). The displayed PL spectrum a sharp of ultraviolet the synthesized near-band-edge transition at 3.52 eV (352 nm). The observedhollow GaN strong spheres band-edge displays a emission sharp emission blue-shifted peak, namely∼ 120 meV relative to that an ultraviolet (UV) near-band-edge transition, at 3.52 eV of bulk GaN (dashed curve(352in nm). the The figure, observed3.40 eV strong). The band-edgeblue-shift should emission be at attributed to a quantum confinement effect. A weak3.52 yellow eV (352 luminescence nm) is blue-shifted transition relative was to observed that of bulk at 2.3 eV (539 nm) in the PL spectrum. The YL emissionGaN at by 2.3 about eV was 120 located meV (13 far nm). below This the should band be edge attributed and was usually contributed to point defects within theto aGaN quantum nanoparticles. confinement effect, because the synthesized

Figure 5. A schematic illustration of the formation process of hollow GaN spheres and hollow nanotubes: a) Generation of nanosized liquid Ga droplets; b) GaN nanocrystal nucleation and growth on the surface of Ga liquid droplets; c) the hollow GaN sphere is formed with small shell size; d,e) at high temperatures the formed hollow GaN spheres aggregate and hollow GaN nanotubes are formed by Figure 7. The room-temperature photoluminescence (PL) spectrum of Figure 12. The room-temperature photoluminescence spectrum of hollow GaN spheres. Dashed line: the coalescence of the nanosized hollow GaN spheres. the synthesized hollow GaN spheres. bulk GaN. Reprinted with permission from [122]. Copyright 2005 Wiley. small 2005, 1, No. 11, 1094 – 1099 2005 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim www.small-journal.com 1097 The PL spectrum of GaN nanotubes shown in Figure6 was almost the same as that of GaN nanospheres. Yellow bands were also observed in PL spectra of GaN nanocomposites [72]. The YL emission should come from the surface defects and vacancies of consisting GaN nanoparticles.

5.2. Raman Scattering

4 The space group of wurtzite GaN is C6v and two formula units are contained in its unit cell. According to the factor group analysis, there are six Raman-active modes, 1A1(TO) + 1A1(LO) + 1E1(TO) + 1E1(LO) + 2E2. Figure 13 shows a typical Raman spectrum of a GaN circular microtubes shown in Figure7. The laser beam was focused on one GaN microtube under an optical microscope and the Raman spectrum was collected from the individual microtube. Among theoretically predicated six active Raman modes, four active modes were observed from the self-assemblies. The peaks at 519, 544, 564, −1 and 719 cm corresponded to the A1(TO), E1(TO), E2(high), and A1(LO) symmetries, respectively. −1 −1 −1 −1 Compared with bulk data (532 cm for A1(TO), 559 cm for E1(TO), 568 cm for E2(high), 734 cm for A1(LO)), the four active Raman modes signiﬁcantly red-shifted to shorter wavenumbers. Two other Raman modes were observed on the low wavenumber side of the A1(LO) mode. The two peaks −1 −1 at 656 cm and 704 cm should be surface optical (SO) phonon modes corresponding to A1 and E1 symmetries respectively. The SO phonon modes should be caused by surface effects of the GaN nanorods. A unusual Raman peak was observed at 414 cm−1 (inset of Figure 13). The peak should be the acoustic overtone from wurtzite GaN nanoparticles. Table2 compares the Raman modes of self-assembled GaN circular microtubes and GaN crystalline bulks. All the Raman-active modes red-shifted compared with crystalline bulks. 142 Y. Lan et al. / Journal of Crystal Growth 415 (2015) 139–145

800

600

400 Cu Intensity (counts)

200 Ga N Cu

0 Intensity (arb. unit) 0510150 1 2 3 4 Energy (kV) Distance (um)

Micromachines 2016, 7, 121 15 of 22

Fig. 3. (a) BF-TEM image of a GaN microtubule. Several typical nanopores are marked by arrows. (b) EDS spectrum of the microtubule in (a). Cu signal coming from the TEM A1(TO), E1(TO),grid. (c) and SAED A1(LO) pattern modes of the of microtubule the self-assemblies in (a). (d) EDS shifted linescan more of Ga thanKα theseacross of the individual microtubule in (a). (e) BF-TEM image of several adjacent nanorods near a nanopore. GaN nanoparticles.(f) SAED of the nanorod circled in (e). (g) HRTEM image of the nanorod circled in (e). (h) IFFT of (g). Inset: FFT of (g). Two green circles mark the diffraction spots used for IFFT. Raman modes(For interpretation of other kinds of the of free-standing references to colorself-assemblies in this ﬁgure also caption, shifted the as the reader circular is referred microtubes. to the web version of this paper.)

25 the condensed droplets is usually 5–50 μm [53], depending on 30 reaction temperatures and substrates. The behavior of gallium is 20 20 very similar to that of other low melting point metals, such as selenium [17]. Gallium droplets evaporate at high temperatures to 10 414 /cm produce gallium vapor during the CVD procedure. Once ammonia 15 0 passes the gallium droplets, gallium vapor would react with 400 500 600 700 800 ammonia to produce GaN nanoparticles (Fig. 6b). The number of 10 the GaN nanoparticles per unit volume is so dense around the gallium droplets that the GaN nanoparticles around the gallium droplets can be considered as a supersaturated aerosol, similar to 5

Raman Intensity (arb.u.) charged nanoparticles in solutions [54] and nanoparticles in vapor phases [55,56]. These supersaturated GaN nanoparticles would 0 spontaneously aggregate into Liesegang rings due to thermody- 450 500 550 600 650 700 750 namic stability, as observed in other kinds of ordered Liesegang Wavenumber (/cm) nanoparticle-rings [54,57]. The proposed growth mechanism is experimentally confirmed by Fig. 4. Raman scattering of a GaN microtubule. ○: experimental; : fitting. Inset: Figure 13. Room-temperature Raman scattering of a GaN circular microtubule. ◦: experimental; SEM−: observation. The middle panel in Fig. 6bshowsanSEMimageof whole range of Raman scattering of the microtubule (red curve) compared with fitting. Inset:that wholeof GaN range crystalline of Ramanfilms scattering (blue curve). of themicrotube (For interpretation (red curve) of compared the references with that to of GaNa typical GaN nanoparticle Liesegang ring with a diameter of several crystallinecolor films in (blue this fi curve).gure caption, Reprinted the withreader permission is referred from to the [123 web]. Copyright version of 2015 this Elsevier. paper.) micrometers. A higher magnification SEM image (bottom panel in Fig. 6b) indicates that the microring consists of many GaN nanopar- Table 2. Raman scattering of hexagonal GaN materials. ticles with a size below 100 nm. Other size GaN rings are also A PL spectrum of zinc-blende microtubes [47] is shown in Fig. 5b observed on the substrates. These surviving GaN Liesegang rings Mode E2 (low) A1(TO) E1(TO) E2(high) SO(A1) SO(E1) A1(LO) E1(LO) Reference for comparison.(cm−1) (cm The−1) PL (cm spectrum−1) (cm− of1) the (cm zinc-blende−1) (cm−1) microtubes (cm−1) (cm− is1) indicate that the Liesegang rings can self-assemble from GaN crystalscentered 144 at 685 533nm. The 561 PL spectrum 569 of - the circular - 735microtubes 743 [nanomaterials128] during the CVD procedures when GaN nanoparticles nanoparticlesblue-shifts † - to higher 528 energy, 552 reflecting 564 the 656 fact that - the wurtzite 730 GaN -are [88] supersaturated enough. microtubes ‡has a wider - band-gap 519 (3.4 544 eV) than 564 the cubic 656 GaN 704 (3.2 eV) 719[50]. - [123]Then, porous GaN microtubules grow on the Liesegang ring SO: surface optical phonon mode. †: crystalline size: ∼12 nm. ‡: crystalline size: ∼100 nm. Growth mechanism: The formation of the porous microtubules templates, as shown in Fig. 6c. Based on the diffusion-limited 6. Potentialcan Applications be explained of GaN by Self-Assemblies the Liesegang phenomenon [51] and diffusion aggregation mechanism, the protruding parts of the GaN micror- limited growth. Gallium has a low melting point of 303 K and a ings can easily attract the GaN nanorods and grow quickly to build GaN self-assembliesboiling point of of 1D 2520 nanomaterials K. Therefore, (nanowires large amounts and nanorods) of gallium have liquid been utilizednew as nanorod-walls from the Liesegang ring edges, leaving the photocatalyticdroplets water splitting can condense [129,130], photovoltaic on quartz substrates devices [131, from132], piezoelectric gaseous gallium nanogeneratorshollow [65], interior. The SEM images in Fig. 6c show tips of two GaN and light-emitting(Fig. 6 diodesa), as observed[63]. Free-standing on silicon GaN substrates self-assemblies[52]. of The 0D nanoparticlesdiameter of havemicrotubules, larger indicating the growth on the Liesegang rings. With surface/volume ratios than these of the GaN 1D nanomaterials. So the reviewed free-standing nanoporous self-assemblies should have wider potential applications.

6.1. Photocatalytic Water Splitting GaN semiconductors have been employed for photoelectrochemical water splitting [133–136] since 2005 [133,134] to generate hydrogen gas. Up to now, GaN nanowire arrays have been utilized to generate hydrogen [129,130,137,138]. The highest incident-photon-to-current-conversion efficiency of 15%–18% [138] was obtained . Nanoporous GaN films were also employed to split water [139]. The nanoporous GaN films showed better efficiency and stability [139]. It was concluded that the large surface area of the porous GaN enhanced hole transport from photoanode to electrolyte and resulted better photovoltaic performances. It was believed [138] that GaN nanostructures possess large surface-to-volume ratios, surface defects, rapid charge carrier separation, and enhanced optical absorption, being a better candidate to split water. Free-standing GaN porous assemblies shown in Figures8–10 have higher surface/volume Micromachines 2016, 7, 121 16 of 22 ratio than all the reported nanowire/nanorod/nanoparticle arrays. Therefor a better photocatalytic performance is expected for free-standing GaN self-assemblies.

6.2. Piezoelectric Nanogenerators GaN nanowire arrays produced output voltage pulses when pressed [65]. Theoretical calculation predicated [140] that piezoelectric constant of GaN nanomaterials increased with decreasing crystalline size of nanomaterials while surface piezoelectric coefﬁcient was constant. Crystalline size of free-standing GaN self-assemblies is several nanometers in three-dimensions, smaller than GaN nanowires that are in nanoscale in two dimensions. Therefore, the free-standing GaN self-assemblies of nanoparticles should have better piezoelectric performances than GaN nanowire arrays.

6.3. Thermoelectric Devices GaN possesses excellent electronic transport properties such as high charge carrier mobility to be thermoelectric materials. In a GaN nanocrystalline ceramic prepared by hot pressing, a reasonably large Seebeck coefﬁcient, S, of −58 µV/K was reported at room temperature [141]. As self-assembly is a promising method for making nanostructured materials in situ, with proper optimization, the self-assembly materials hold promise for high efﬁciency downscaled thermoelectric devices [142]. Potential thermoelectric applications of the GaN-based low dimensional nanomaterials that are formed by self-assembly techniques are of great interest because of the possibility to be shaped into devices and circuits, and because of the potential for direct integration of microcooler/power generators with various electronic devices. Although to date there have not been reports on thermoelectric applications of self-assembled GaN and GaN-based nanomaterials, it is hoped that information on the improved thermoelectric properties of doped GaN-based bulk and low-dimensional nanomaterials present here may stimulate such development.

6.4. Other Potential Applications in Renewable Energy GaN self-assemblies reviewed here are nanoporous and consisted of nanoparticles, holding novel optical properties. This kinds of nanomaterials should also have applications in batteries, capacitors, and solar cells. However, it is difﬁculty to massively produce such kinds of porous nanostructures. Few researches on renewable energy have been reported. More potential applications of the free-standing nanoporous nanomaterials are expected in the coming years.

7. Conclusions The syntheses and physical properties of hexagonal GaN nanoparticles are brieﬂy reviewed. These novel GaN nanoparticles can be self-assembled into free-standing nanospheres, nanotubes, microtubes, and composite bulks without any templates. The mechanism and physical properties of these self-assemblies are discussed. The potential applications of the assemblies are expected based on their high surface/volume ratios and tunable band-gaps.

Acknowledgments: The author Yucheng Lan acknowledges the financial support by the Defense Threat Reduction Agency under Grant HDTRA122221, the support from US Department of Energy under the Contract DE-NA0000720 through the program of S. P. Massie Chair of Excellence. Author Contributions: Jianye Li contributed to nanoparticles, Yucheng Lan and Abdellah Lisfi to selfassemblies, Winnie Wong-Ng drafted potential applications, Rola M. Derbeshi collected references, Yucheng Lan and Jiang Li contributed to organization of the review and potentials in renewable energy. Conflicts of Interest: The authors declare no conflict of interest. Micromachines 2016, 7, 121 17 of 22

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