Nanohomojunction(GaN)andNanoheterojunction(InN)Nanorods on One-Dimensional GaN Nanowire Substrates** By Zon-Huang Lan, Chi-Hui Liang, Chih-Wei Hsu, Chien-Ting Wu, Huang-Min Lin, Sandip Dhara, Kuei-Hsien Chen,* Li-Chyong Chen,and Chia-Chun Chen 1. Introduction In recent advances in assembling nanostructures, complex device functionality has been achieved in only a few selected systems. To realize the potential inherent in nanodevices, new methods of assembling a wide range of dissimilar elements, connecting the resulting complex structures, and integrating themintosystemsmustbedeveloped.Individualsemiconduct- ing nanowires (NWs) have already been configured as bio- or chemical sensors. [1] More sophisticated light-emitting diodes [2] and complementary and diode logic devices have also been fabricated from semiconducting NWs. [3] Theassemblyofsemi- conducting NWs into nanoscale devices and circuits could en- able diverse applications in nanoelectronics. [4] There is there- fore a need for controlling these structures and their diverse properties. Heterostructure formation in one-dimensional (1D) systems is important for their potential applications as efficient light- emitting sources and better thermoelectric devices. [5] Previous studies on semiconducting NWs or nanotubes (NTs) have in- variably dealt with homogeneous systems, with a few excep- tions of heterostructure formation. [6] Semiconducting nanoho- mojunction (SnO) [7] and nanoheterojunction (ZnO on In 2 O 3 ) NWs [8] in the form of nanobrushes have been reported pre- viously.Infact,thetwo-dimensional(2D)semiconductorinter- face is ubiquitous in optoelectronic devices such as light-emit- ting diodes, quantum-cascade lasers, and transistors. While thereareanumberofwell-developedtechniquesforthefabri- cation of thin film (2D) heterojunctions, [9] a general synthetic scheme for heterojunction formation in 1D nanostructures is currentlystilllacking. In the present report, we demonstrate the hierarchical growth of semiconducting GaN and InN nanorods (NRs) in theformofnanobrusheswithfour-andtwofoldstructuralsym- metriesona1DsubstrateofGaNNW.Thenanohomojunction (GaN on GaN) and nanoheterojunction (InN on GaN) NRs are produced by two-step growth in a vapor±liquid±solid (VLS) mechanism. [10] In the first step, Ga 2 O 3 nanoribbons are formedbyachemicalvapordeposition(CVD)techniqueusing NiasthecatalystintheVLSprocess.TheGa 2 O 3 nanoribbons are converted by ammonia reduction to GaN NWs, which are thenusedasthe1Dsubstrate.Inthesecondstep,Auisusedas thecatalystintheVLSprocessfortheformationofnanoheter- ojunctions and nanohomojunctions using a CVD technique. In principle, this approach can be successfully implemented if the conditionsofthesecondreactionstepdonotaffecttheproduct of the first step. The design, synthesis, and characterization of surface-modified nanostructures are of fundamental impor- tanceincontrollingthemesoscopicpropertiesofnewmaterials and in developing new tools for nanofabrication. These kinds of networked nanobrushes suggest potential applications rang- ing from nanodevices to an artificial nerve network if one can functionalize the nanocluster to act as a receiver. The concept of using NW field-effect transistors (FETs) modified with re- Adv. Funct. Mater. 2004, 14, No. 3, March DOI: 10.1002/adfm.200304403  2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 233 ± [*] Dr. K. H. Chen, Z. H. Lan, C. T. Wu, Dr. S. Dhara [+] Institute of Atomic and Molecular Sciences, Academia Sinica Taipei106(Taiwan) E-mail: chenkh@po.iams.sinica.edu.tw C. H. Liang, Dr. L. C. Chen Center for Condensed Matter Sciences National Taiwan University Taipei106(Taiwan) H. M. Lin, C. W. Hsu, Dr. C. C. Chen Department of Chemistry National Taiwan Normal University Taipei106(Taiwan) [+] Author is on leave from the Materials Science Division, Indira Gandhi Center for Atomic Research, Kalpakkam 603 102 (India) [**] Financial support from the National Science Council, Taiwan, under contracts NSC 91-2120-M-001-001, NSC 91-2120-M-003-001 and NSC 91-2112-M-002-055, Taiwan's Ministry of Education (No. 90-N- FA01-2-4-5), and NTNU ORD 92-03 is gratefully acknowledged. Theformationofhomojunctionsandheterojunctionsontwo-dimensional(2D)substratesplaysakeyroleinthedeviceperfor- manceofthinfilms.Acceleratingtheprogressofdevicefabricationinnanowires(NWs)alsonecessitatesasimilarunderstand- ing in the one-dimensional (1D) system. Nanohomojunction (GaN on GaN) and nanoheterojunction (InN on GaN) nanorods (NRs)wereformedinatwo-stepgrowthprocessbyavapor±liquid±solid(VLS)mechanism.Ga 2 O 3 nanoribbons were formed usingNiascatalystinachemicalvapordeposition(CVD)techniqueandthencompletelyconvertedtoGaNNWswithNH 3 as reactantgas.AnAucatalystisusedinthesecondstepoftheVLSprocesstogrowGaNandInNNRsonGaNNWsusingCVD techniques.Amorphologicalstudyshowedtheformationofnanobrusheswithdifferentstructuralsymmetriesandsub-symme- triesinbothhomogeneousandheterogeneoussystems.Structuralcharacterizationsshowednearlydefect-freegrowthofnano- homojunction(GaN)andnanoheterojunction(InN)NRson1DGaNNWsubstrates. FULL PAPER