Abstract: Low-dimensional ZnO materials, (1-dimensional ZnO nanorods (NRs) and 2-dimensional ZnO films) weregrown perpendicularly on graphite substrates. The crystalline structure, morphology, and optical properties of theas-grown low-dimensional ZnO materials were investigated with X-ray diffraction (XRD), field effect scanning elec-tron microscope (FESEM), photoluminescence (PL), and the relative reflection spectra measurements. The high opti-cal qualities of borth ZnO NRs and ZnO films on graphite substrates were demonstrated by the dominant near-bandedge emission and nearly undetectable deep level emissions of photoluminescence spectra under room temperature.The extremely low average reflectance was obtained from the low-dimensional ZnO materials/graphite structures inthe spectra range from 300 nm to 800 nm, indicating that the obtained low-dimensional ZnO materials/graphite struc-tures have significant opportunity for potential application in high-performance photovoltaic devices.

Key words: ZnO; graphite; photoluminescence; high power optoelectrics devices

As semiconductor devices become smaller dimensionsof new electronics components are reaching nanometerscale. Understanding the fundamental properties of thesenanostructure materials will provide opportunities to de-sign advanced materials and to fabricate novelnano-devices for future applications. ZnO is probably therichest family of low-dimensional structures among allmaterials, which exhibits the most splendid and abundantconfigurations of nanostructures. The large direct bandgap of 3.37 eV along with the large exciton binding energyof 60 meV, make ZnO a strong candidate for the next gen-eration of ultraviolet (UV) light-emitting diode (LED) andlasing devices operating at high temperatures and in harshenvironments [1-2] . Recent improvements in the control ofbackground conductivity of ZnO and demonstrations ofp-type doping have intensified interest in this materialfor applications in optoelectronic field [3] . As a material,ZnO can be grown as bulks, thin films and nanostruc-tures. Especially, low-dimensional ZnO materials are ofparticular interest due to its excellent material character-ristics and potential applications in constructing novel performance optoelectronic and electronic devices. De-spite the considerable interests and rapid developments inlow-dimensional ZnO materials, some essential issueswith regards to the fundamental properties deserve furtherinvestigation. For example, an important and key chal-lenge for the technological applications of high-powerZnO based devices is the severe heat dissipation problem,which might significantly affect the power persistence of ahigh-power device based on low- dimensional ZnO mate-rials [4] . So far, a variety of methods have been employed tofabricate high-quality low- dimensional ZnO materials onvarious single-crystal substrates, such as sapphire, ZnO,and Si [5-7] . Nevertheless, the heat dissipation performancewould not be satisfied due to the relatively high thermalresistance of these substrates. In addition, for some specialapplications such as large area foldable and high-powerdevices, it is necessary to transfer crystalline ZnO ontoforeign substrates, such as flexible plastic or metal sub-strates [8-9] . However, it is difficult to separate the ZnO NRsfrom the above mentioned single-crystal substrate becauseof strong bonding between them, this presents one of the major limits for such applications. Graphite substrates areconsidered to be a good solution to this issue, and the ad-vantage of graphite lie in its low cost, non-toxic, excellentmechanical and chemical stability, especially the superiorelectrical and thermal conductivity even higher than cop-per [10] , as well as the potential advantage for transferableoptoelectronics devices since it consists of multi-layersystem with nearly decoupled 2D graphene planes. Allthese features of the graphite substrate provide significantopportunities for fabricating various transferable and lowthermal resistance high-power electronic and optoelectronicdevices. In addition, the excellent heat dissipation perform-ance of the graphite-insulator-semiconductor (GIS) diodecompared with conventional sapphire based devices hasbeen systematically demonstrated in our previous work,indicating that the GIS structure would be of special interestfor the development of high-power semiconductor deviceswith sufficient power durability [11] .

In this study, low-dimensional ZnO materials, including1-dimensional ZnO nanorods (NRs) and 2-dimensionalZnO films were grown perpendicularly on graphite sub-strates. Considering the excellent material characteristics oflow-dimensional ZnO materials and the versatile and fas-cinating features of graphite substrates, the achievementsmake it possible for the development of high performanceZnO based optoelectronic devices with sufficient powerdurability.

1 Experimental

ZnO NRs were grown on graphite substrates by thesimple wet chemical base deposition (CBD) method,which was a high performance growth technique forZnO nanostructures due to its obvious advantages oflow-cost, low temperature operation and environmentalfriendliness [12] . Detailed process by CBD technique canbe found in our previous work [12] . ZnO films weregrown with the relatively simple process of ultrasonicspray pyrolysis (USP), which was successfully em-ployed to grow p-type ZnO (ZnMgO) films and ZnOp-n homojunction LEDs in our previous study [13] .

The morphology and crystal structure of the obtainedZnO NRs and films were investigated by field effect scan-ning electron microscope (FESEM) on JEOL JSM 6700F,and X-ray diffraction (XRD) on SHIMADZU XRD-6000.Photoluminescence (PL) measurements were performed atroom temperature by a Jobin Yvon HR320 spectrometerusing a He-Cd laser (30 mW) with an excitation wave-length of 325 nm. The relative reflection spectra weremeasured on UV-VIS-NIR spectropho- tometer (UV-3600)with integrated sphere.

2 Results and discussion

2.1 Structure characters of ZnO nanorods andfilms on graphite substrates

Figure 1 shows the typical XRD patterns of low-dimensionalZnO materials grown on graphite substrates. As shown inFig. 1(a), a dominant ZnO (002) diffraction peak at34.511º companied with a high intensity graphite (002)diffraction peak at 26.476º are observed, no peak fromother compounds is detected besides that of ZnO. Thismeans that the c-axis preferentially oriented ZnO NRs arevertical to the graphite substrate. As shown in Fig. 1(b),three diffraction peaks are obviously observed and can allbe indexed to ZnO (100), (002) and (101). This indicatesthat ZnO films deposited on graphite substrate exhibitpolycrystalline structure with no preferred orientation. Itshould be noted that it is nearly impossible to grow epi-taxial ZnO single crystal film directly on amorphousgraphite substrate due to the extremely large lattice mis-match between ZnO and amorphous graphite substrate aswell as the relatively low growth temperature of USP,therefore only wurtzite ZnO polycrystalline films are suc-cessfully obtained and will be employed as buffer layer fortransferable optoelectronics devices.

2.2 Morphology of ZnO nanorods and filmson graphite substrates

Figure 2 shows the typical top-view and correspondingcross-sectional SEM images of low-dimensional ZnO ma-terials grown on graphite substrates. ZnO NRs grown withCBD method at 95 for 3 ℃ h and ZnO films grown withUSP technique at 500 for 5 min. As shown in  ℃ Fig. 2(a),it is clearly seen that high density and random-alignedZnO NRs with the average diameter and length to be ~55nm and ~2.3 μm are synthesized on the graphite sub-strates. Furthermore, it shows that each ZnO NR has auniform diameter along its entire length, indicating thatthe growth anisotropy is constantly maintained. More-over, it should be noted that nearly all the individual ZnONRs present well defined hexagonal prism shape withhomogeneous diameter and smooth side facets, whichwill be particular beneficial for the formation of a naturalwhispering-galley-mode (WGM) nanocavity based onthe total internal reflection at the cavity boundary. As forthe ZnO films grown with USP technique (Fig. 2(b)), itcan be seen that the ZnO films on graphite substratespresent an irregular surface structure comprised of py- ramidal-shaped ZnO nanosheets. The correspondingcross-section SEM images show that the ZnO films pre-sent uniform thickness and dense distribution.

2.3 Optical properties of ZnO nanorods andfilms on graphite substrates

Optical properties of ZnO NRs are important formany of their technological applications. Figure 3 pre-sents the typical room-temperature PL spectra oflow-dimensional ZnO materials grown on graphite sub-strates. Generally, the UV emission in ZnO PL spectra isaccepted as the near-band-edge (NBE) emission whichhas an exciton nature [14] . On the other hand, the greenemission band in ZnO PL spectra was usually observedfor most ZnO samples reported in literature, which wasbelieved to be closely related to the defect level inducedby the defects of O vacancies, Zn interstitials or theircomplexes [15-16] . In this study, however, it should be notedthat only strong NBE UV emission peaks were observed-for the low-dimensional ZnO samples grown on graph-ite,yet the usually observed defect related deep level emis-sions were nearly undetectable, indicating high opticalquality low-dimensional ZnO materials were successfully achieved  via  this  simple  USP  approach.  Suchlow-dimensional ZnO materials with high optical qualityare synthesized only by physical techniques like molecularbeam epitaxy (MBE), metal-organic chemical vapor deposi-tion (MOCVD), and gold-catalyzed vapor-phase-transport(VPT). However, those are expensive and energy con-suming processes since they are operated under extremeconditions. Here, high optical qualities low-dimensionalZnO materials were fabricated by simple low-temperatureprocess.

Figure 4 shows the measured relative reflection spectrafrom 300 nm to 800 nm of graphite substrates (black solidline), the ZnO NRs/graphite, and the ZnO films/graphitesamples (red solid line), respectively. The extremely lowaverage reflectance of 0.196% and 3.7625% are obtained forthe ZnO NRs/graphite and ZnO films/graphite structures inthe spectra range from 300 nm to 800 nm. The lower re-flectance compared with graphite substrates should beattributed to the enhanced incident light trapping by the scattering effect of surface nanostructure [17] . Given thefact that the transmission of graphite substrates was nearlyzero, the correspondingly absorption in solar spectra would be rather high. Therefore, the reported ZnO NRs/graphiteand ZnO films/graphite structures have significant opportu-nity for potential application in commercial photovoltaicdevices due to its distinctive low reflectivity and corre-spondingly high absorption in solar spectra.

3 Conclusions

High optical quality low-dimensional ZnO materials,including 1-dimensional ZnO nanorods (NRs) and2-dimensional ZnO films, were grown perpendicularly ongraphite substrates. NBE UV emission were observed inroom-temperature photoluminescence spectra for the an-nealed samples, yet the usually observed defect relateddeep level emissions were nearly undetectable, indicatinghigh optical quality low-dimensional ZnO materials couldbe achieved on graphite substrates. Considering the excel-lent material characteristics of low-dimensional ZnO andthe versatile and fascinating features of graphite substrates,the achievement makes it possible for the development ofhigh performance ZnO based devices with sufficientpower durability, especially for commercial photovoltaicdevices due to its distinctive low reflectivity and corre-spondingly high absorption in solar spectra.

References:[1] Liang H K, Yu S F, Yang H Y. ZnO random laser diode arrays forstable single-mode operation at high power. Appl. Phys. Lett., 2010,97(24): 241107–1–3.[2] Bian J M, Liu W F, Liang H W, et al. Room temperature elec-troluminescence from the n-ZnMgO/ZnO/p-ZnMgO heterojunctiondevice grown by ultrasonic spray pyrolysis. Chem. Phys. Lett.,2006, 430(1/2/3): 183–187.[3] Norton D P, Heo Y W, Ivill M P, et al. ZnO: growth, doping &processing. Mater. Today, 2004, 7(6): 34–40.[4] Dürr A C, Schreiber F, Kelsch M, et al. Morphology and thermalstability of metal contacts on crystalline organic thin films. Adv.Mater., 2002, 14(13/14): 961–963.[5] Kaidashev E M, Lorenz M, Wenckstern H V, et al. High electronmobility of epitaxial ZnO thin films on c-plane sapphire grown bymultistep pulsed-laser deposition. Appl. Phys. Lett., 2003, 82(22):3901–3903.[6] Xu W Z, Ye Z Z, Zeng Y J, et al. ZnO light-emitting diode grownby plasma-assisted metal organic chemical vapor deposition. Appl.Phys. Lett., 2006, 88(17): 173506–1–3.[7] Bian J M, Li X M, Chen L D, et al. Properties of undoped n-typeZnO film and N-In codoped p-type ZnO film deposited by ultrasonic spray pyrolysis. Chem. Phys. Lett., 2004, 393(1/2/3): 256–259.[8] Chung K, Lee C H, Yi G C. Transferable GaN layers grown onZnO-coated graphene layers for optoelectronic devices. Science,2010, 330(6004): 655–657.[9] Lee C H, Kim S J, Oh Y, et al. Use of laser lift-off for flexible de-vice applications. J. Appl. Phys., 2010, 108(10): 102814–1–5.[10] Bonnissel M, Luo L, Tondeur D. Compacted exfoliated naturalgraphite as heat conduction medium. Carbon, 2001, 39(14):2151–2161.[11] Zhang Z K, Bian J M, Sun J C, et al. ZnO-based graphite- insula-tor-semiconductor diode for transferable and low thermal resis-tance high-power devices. Appl. Phys. Lett., 2012, 101(5):052108–1–4.[12] Li Q W, Bian J M, Sun J C, et al. Controllable growth ofwell-aligned ZnO nanorod arrays by low-temperature wet chemical bath deposition method. Appl. Surf. Sci., 2010, 256(6): 1698–1702.[13] Bian J M, Liu W F, Sun J C, et al. Synthesis and defect-relatedemission of ZnO based light emitting device with homo-and het-erostructure. J. Mater. Process. Tech., 2007, 184(1/2/3):451–454.[14] Sun Y, Ketterson J B, Wong G K L. Excitonic gain and stimulatedultraviolet emission in nanocrystalline zinc-oxide powder. Appl.Phys. Lett., 2000, 77(15): 2322–2324.[15] Look D C, Hemsky J W, Sizelove J R. Residual native shallowdonor in ZnO. Phys. Rev. Lett., 1999, 82(12): 2552–2555.[16] Shan F K, Liu G X, Lee W J, et al. The role of oxygen vacancies inepitaxial-deposited ZnO thin films. J. Appl. Phys., 2007, 101(5):053106–1–8.[17] Garnett E, Yang P D. Light trapping in silicon nanowire solar cells.Nano Lett., 2010, 10(3): 1082–1087.