Growth and transmission electron microscopy studies of nanomaterials 3 4

Chapter Growth of boron nitride nanostructural materials Chapter Growth of boron nitride nanostructural materials In this chapter, the nucleation and growth of boron nitride nanomaterials in microwave plasma enhanced CVD system and in-situ TEM is presented in detail 3.1 Introduction The growth chemistry and atomic structure of BN nanostructures have attracted much attention because the wide band gap exhibited by BN suggests interesting applications in novel UV lasers and low-k materials [1-3] Following the discovery of the carbon nanotube by Iijima [4], similar structures have been proposed and discovered for BN [5] Compared with the carbon nanotubes, BN nanotubes have been predicted to display a bandgap of roughly 5.5 eV, independent of their chirality together with high ultimate strength and oxidation resistance [5, 6] Thermal stability and oxidation resistance make BN a suitable material to store and protect some air-sensitive metals inside, such as pure Co and Fe [7] 3.2 Motivation BN nanotubes have been synthesized by a range of methods from arc discharge [8, 9], chemical vapor deposition [10, 11] to solid-state ball milling methods followed by annealing at high temperature [12, 13] Non-catalytic growth 56 Chapter Growth of boron nitride nanostructural materials methods have also been demonstrated [14-16] Compared with the growth condition required for carbon nanotubes, most of these methods require thermal conditions higher than 1200 ˚C and the yield of BN nanomaterials is often low The growth temperatures required for BN synthesis are often too high to be compatible with microelectronic processing on conventional semiconductor substrates Hence, the properties and technological applications of BN nanotubes, nanowires and nanospheres have not been fully investigated due to the lack of these materials in sufficient quantity Narita and Oku [17-19] have studied the arc-melting of a sequence of borides such as TiB2, VB2, NbB2, LaB6 for the production of BN nanocapsules and nanotubes The formation enthalpies of BN from the respective borides were considered as the key factor in catalyst design However another important factor is the composition of the boride phase in the design of the catalyst, since this will influence the eutectic melting point A lower eutectic melting point will promote the ready formation of a molten phase to initiate the VLS mechanism [20], thus allowing the phase segregation of BN nanomaterials at lower temperatures However, the direct plasma nitridation of iron borides at moderately low temperatures, i.e 200 0.5–1 µm Few Fig 3.7 Microspheres 0.5 µm Many Fig 3.6 Table 3.1 summarizes the apparent relationship between the size of the FeB particle and the BN nanomaterial that ensued from it, as was observed in this work It is more difficult for the larger particles to achieve fluid-like motion compared to the smaller particles So if the smaller FeB particles aggregate to form a FeB particle larger than 200 nm, the mobility of this particle is restricted and only an outer BN microcapsule grows on it Nanotubules are generally observed to grow from FeB 63 Chapter Growth of boron nitride nanostructural materials particles with sizes in the range of 50–200 nm, because these particles melt and flow readily, and trace out nanotubules with bamboo-like segments in its motion What is interesting are FeB particles with sizes ranging from 50 to 100 nm, smaller BN nanotubes and nanocapsules are readily created from these, and in addition, these FeB particles readily melt and recrystallize into crystalline Fe nanowires ensheathed by BN A detailed discussion of the various structural polytypes follows below Figure 3.3 TEM images of BN nanotubes recovered from the FeB nanoparticles after nitrogen-plasma treatment at 850 ˚C Some BN nanotubes can be observed in the nitridation products, as shown in figure 3.3a These are BN nanotubes with diameters of 20-30 nm and length to 200 nm Higher magnification images were recorded in order to observe the internal structure of these nanotubes Unfortunately, the low contrast between the BN nanotubes and the carbon support film leads to poor quality images However, we can still see tat the walls of the BN nanotube are poor by crystalline with many 64 Chapter Growth of boron nitride nanostructural materials defects which make the BN nanotubes unstable under the strong electron beam These nanotubes may have evolved from a partial crystalline BN film during the nitridation process For the growth of BN nanotubes, a high temperature and a slow cooling rate would be needed In our plasma system, 850 ˚C may not be a high enough temperature to form high quality BN nanotubes Figure 3.4 TEM images of BN nanocapsules For FeB particles with sizes between 50 and 100 nm, nanotubules or nanocapsules are readily formed from them We can observe the Fe or FeB-FeN nano-sized particles encapsulated in the BN nanocages Isolated FeB particles that escaped sintering afforded an interesting observation point for the growth of BN nanocages Figure 3.4 shows TEM images of BN nanocages consisting of multi-layered hexagonal BN with some FeB particles leaving the BN nanocages 65 Chapter Mechanistic Study of Zinc Sulfide Nanowires 520 nm is present as shown in figure 4.11(a), this has been previously associated with the Au-Zn-S luminescent centre [1] Figure 4.11(b) shows the room temperature PL spectra of these nanocrystalline ZnO nanowires produced by atomic O beam treatment at 700 ˚C A single peak at 380 nm due to free-exciton recombination [4, 42] can be seen The absence of defect-related peaks suggests that the ZnO nanocrystals obtained by atomic O beam treatment are of high-quality, with few vacancies or interstitial-related traps Therefore at 700 ˚C, atomic O can effect a highly efficient conversion of ZnS into ZnO 515 nm 380 nm 682 nm PL intensity a b c d 400 500 600 700 800 Wavelength (nm) Figure 4.11 Photoluminescence spectra from (a) ZnS nanowires, (b) ZnO nanowires after atomic oxygen oxidation at 700 ˚C, (c) ZnO nanowires after molecular oxygen oxidation at 700 ˚C and (d) ZnO nanotubules after atomic oxygen oxidation at 500 ˚C 113 Chapter Mechanistic Study of Zinc Sulfide Nanowires In comparison, experiments on the molecular oxidation of ZnS nanowires at a pressure of 10 torr reveal that the resultant ZnO wires have more defects This is demonstrated by the photoluminescence spectra of ZnS nanowires subjected to molecular oxidation shown in figure 4.11(c), which shows that in addition to the exciton peak, broad peaks at 510 nm and 680 nm [43, 44] are present, which may be attributed to singly ionized oxygen vacancies and interstitial zinc The reaction efficiency of atomic O with ZnS nanowires is much higher than with molecular O2 since pre-dissociation is not needed, and direct diffusion into the ZnS matrix can occur The PL spectra of the ZnO nanotubules produced by atomic oxidation at 500 ˚C is shown in figure 4.11(d), where a strong defected-related emission at 682 nm due to interstitial zinc can be seen, a characteristic of the incomplete structural reorganization at the lower temperature 4.5.2 Oxidation mechanism of ZnS to ZnO ZnS nanowires 700 ˚C 500 ˚C ZnO nanocrystalline wires ZnO nanotubules Our studies show that two types of products are obtained here following atomic O beam treatment of the ZnS nanowire, depending on the temperature At the 114 Chapter Mechanistic Study of Zinc Sulfide Nanowires higher temperature of 700 ˚C, there is a transformation into optically active (380 nm, emission caused by free exciton bound to neutral acceptors), nanocrystalline ZnO wire, with a notable reduction in diameter compared to the original ZnS nanowire Whereas at a temperature of 500 ˚C, a hollowing of the core to form ZnO nanotubules occurs We propose a mechanism to explain the different reaction products based on the competing rate between the oxidation and evaporation of ZnS in vacuum ZnS has a partial vapour pressure of 10-4 torr at 800 ˚C and atmospheric conditions, whereas ZnO requires temperatures in excess of 1800 ˚C to reach the equivalent partial pressures At 700 ˚C, atomic O can diffuse rapidly into the ZnS nanowire to produce complete oxidation of the wire and prevents its evaporation Our periodic bulk calculations show that there is a contraction in the average unit cell volume following the alloying of ZnS with increasing concentration of O to form ZnSxO1-x Figure 4.12 shows the equilibrium geometries of the ZnSxO1-x after the progressive substitution of S atoms by O atoms The structural parameters around the substituted O atoms are highlighted It is found that both the bond lengths of the Zn-O and the Zn-O-Zn bond angles decrease in ZnSxO1-x when compared to ZnS Such distortion of the lattice introduces considerable compressive stress which is relieved by the breaking up of the lattice into ZnO nanocrystals Numerous voids are created when the lattice breaks and contracts into nanocrystals, these will facilitate the further diffusion of oxygen into the core and enhance the oxidation 115 Chapter Mechanistic Study of Zinc Sulfide Nanowires process When the entire assembly is thermally activated, the ZnO nanocrystals will agglomerate to form a reconstituted ZnO nanowire with a diameter that is reduced by at least 20% when compared to ZnS due to the lattice contraction Figure 4.12 The model and selected structure parameters of ZnSxO1-x The bond lengths are given in Å 116 Chapter Mechanistic Study of Zinc Sulfide Nanowires Figure 4.13 The side view of the surface slabs of the optimized S-terminated (0001) ZnS surface and the O-substituted (0001) ZnS surface The bond lengths are given in Å At 500 ˚C, the diffusion of O into the bulk of ZnS is less efficient In this case, the oxidation is limited to the peripheral walls at first to form a ZnO shell which is thermally more stable than ZnS at this temperature Due to the slower rate of inward oxidation, the ZnS in the core evaporates at 500 ˚C in vacuum and the core collapses into voids, thus producing a nanotubule with a ZnO shell The diameter of the ZnO nanotubule is similar to the initial ZnS nanowire from which it derives because complete structural relaxation is prevented at this temperature 117 Chapter Mechanistic Study of Zinc Sulfide Nanowires Figure 4.13 shows the side view of the surface slabs of the optimized S-terminated (0001) ZnS surface, and the O-substituted (0001) ZnS surface The surface slab calculations reveal that the replacement of S by O on the surface results in around 6% surface bond contraction and a 17% reduction in the O-Zn-S bond angle when compared to the S-Zn-S bond angle in both two O- substituted bilayers In addition, compared to the S-terminated surface, the distance between the first bilayer and the second bilayer in the O-substituted surface is reduced by 17% The growth of thicker ZnO layers will result in a build up of compressive stress on the surface In the case of ZnS nanowires undergoing oxidation radially and tangentially around the walls, the resultant elastic instability of the ZnO-ZnS interface results in buckling and curling of the ZnO nanotubules When the ZnO nanotubules are subjected to high-energy electron beam irradiation, they readily change to form a loose agglomeration of ZnO nanocrystals whilst maintaining the outward morphology of a wire The diameter of the reconstituted ZnO nanotubule is cracked and reduced by almost 40% compared to the original ZnS nanowire This is seen from the TEM image of the re-constituted ZnO nanocrystalline wire in figure 4.9(c), generated from the ZnO nanotubule after 300 KeV electron beam irradiation Our results suggest the possibility of using a two-step strategy to restructure the diameter of the wire template by first going through the "hollowing out" process to form the nanotubule, and then a second 118 Chapter Mechanistic Study of Zinc Sulfide Nanowires structural relaxation process to collapse the voids to form a more compact but considerably thinner nanocrystalline ZnO wire This structural relaxation can be achieved by annealing the ZnO nanotubule at a higher temperature in oxygen, which has the added advantage of annealing out the defects in the nanocrystalline ZnO wire to form optically active nanocrystalline ZnO wire Figure 4.14 TEM images of ZnS nanowires after annealing in-situ for (a) s, (b) 80 s, (c) 1020 s and (d) 1500 s The formation of nanotubes from ZnS nanowires can also occur by electron beam irradiation under oxygen-free conditions Figure 4.14 shows a series of TEM 119 Chapter Mechanistic Study of Zinc Sulfide Nanowires images taken as a function of time while a ZnS nanowire is being irradiated with 200 kV electrons in a background pressure of 1×10-8 torr and a temperature of 300 ˚C Bright field images were taken every seconds to record the structural changes taking place during the heating process and images recorded after 0, 400, 1020 and 1500 seconds are shown in figure 4.14 It can be seen that the ZnS nanowire becomes increasingly porous with increasing electron beam irradiation time, while the edges of the nanowire remain seemingly intact This transformation only occurred when the sample was both heated to 300 ˚C and electron beam irradiated Interaction with charged particles can produce various effects on the material being irradiated For small energy transfers, electronic excitation of valence electrons into the conduction band creates excited states Excited states can lead to local chemical modification by atomic-bonding instabilities and rearrangement ZnO nanocrystals have previously been observed to form from ZnS by electron beam irradiation [45] In addition, ZnO is known to be damaged under an electron beam by the release of O leaving behind Zn [46] The relative stability of the peripheral regions compared to the core may be due to the presence of a thermally more stable ZnO layer present as an outer sheath on the ZnS 120 N(E)/E (a.u.) Chapter Mechanistic Study of Zinc Sulfide Nanowires (b) (a) 1016 1018 1020 1022 1024 1026 1028 Binding Energy (eV) Figure 4.15 XPS spectra of (a) ZnS nanowires and (b) fully oxidized ZnO nanowires from ZnS under an environment of atomic oxygen at 700 ˚C To prove this hypothesis, X-ray photoelectron spectroscopy analysis was applied to characterize the surface chemical states of Zn atoms on the ZnS and ZnO nanowires The Zn 2p3/2 peak for synthesized ZnS nanowires and fully oxidized ZnO are shown in figure 4.15 respectively From this figure, the Zn 2p3/2 peak of pure ZnO is symmetric and located at 1022.1 eV, which is in excellent agreement with the published data [47] However, the Zn 2p3/2 peak of the untreated ZnS is not symmetric, and can be deconvoluted into two peaks using the Gaussian method The deconvoluted peak is a combined Zn 2p3/2 peak from the ZnS nanowires and a peak 121 Chapter Mechanistic Study of Zinc Sulfide Nanowires similar to ZnO The presence of two chemical bonding environments for Zn clearly indicates several layers of ZnO outer coating have formed during the growth of ZnS nanowires These oxidized layers may come from the ZnS nanowires growth process When ZnS segregates from the Au-ZnS alloy, the outer layers of wire are very reactive and can be oxidized by a small amount of O2 present in the vacuum XPS analysis of the original ZnS nanowires shows the presence of surface oxides on the wire and we attribute this to the oxidation of the ZnS surface during growth of the ZnS nanowires This hollowing process further proves the oxidation mechanism proposed above Atomic O in the gas phase diffuses into the outer layer and effects a conversion of this region into a crystalline ZnO shell The ZnS in the core evaporates in high vacuum and cracks due to the compressive force caused by the formation of ZnO, which results in material loss from the core The outer shell of ZnO is stable in the oxidizing environment and acts as a reverse template for the deposition of ZnO from volatile ZnS vapour and oxygen atoms Therefore, there is an outward-driven radial movement of ZnS onto the inner face of the ZnO shell to form ZnO, contributing to the hollowing out of the core 122 Chapter Mechanistic Study of Zinc Sulfide Nanowires 4.6 Conclusion ZnS nanowires can be successfully grown on Si substrates using Au as a catalyst AFM images show the growth is follow the Vapor-Liquid-Solid mechanism Direct morphology transfer between ZnS nanowires and nanocrystalline ZnO nanowires or nanotubules can be obtained by atomic O beam treatment in vacuum The ZnO nanocrystalline wires are optically active, with exciton-derived room temperature photoluminescence following atomic O beam-induced conversion at 700 ˚C A lowering of the oxidation temperature favors the formation of ZnO nanotubules which provide a one-dimensional scaffold for the suspension of very fine ZnO nanocrystals at the core The “hollowing” process of ZnO nanotubule formation, followed by structural relaxation during high temperature annealing, can potentially provide an effective strategy to form very thin nanocrystalline ZnO nanowires ZnO nanocrystalline wires may show interesting properties as an active random medium in stimulated UV lasing [48], which will be the subject of further investigations 123 Chapter Mechanistic Study of Zinc Sulfide Nanowires References [1] Wang, Y.W.; 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Saturation and growth 43 0 nm exposed to ZnS vapor (920 °C) 40 0nm exposed to ZnS vapor (870 °C) Figure 4. 4 AFM images of different stages during the ZnS nanowire growth process Figure 4. 4a -4. 4d show... ensure the isolation of the slabs 4. 4 Growth mechanism of ZnS nanowires 4. 4.1 Crystal structure and morphology of ZnS nanowires After deposition of the ZnS powder, one layer of white film on the