Editing Photoconductivity (section)

==Negative photoconductivity==
Some materials exhibit deterioration in photoconductivity upon exposure to illumination.<ref name="Joshi1990">{{cite book|author=N V Joshi|title=Photoconductivity: Art: Science & Technology|url=https://books.google.com/books?id=lv-Nb5-H3pQC&pg=PA272|date=25 May 1990|publisher=CRC Press|isbn=978-0-8247-8321-1}}</ref> One prominent example is [[hydrogenated amorphous silicon]] (a-Si:H) in which a metastable reduction in photoconductivity is observable<ref name="StaeblerWronski1977">{{cite journal|last1=Staebler|first1=D. L.|last2=Wronski|first2=C. R.|title=Reversible conductivity changes in discharge-produced amorphous Si|journal=Applied Physics Letters|volume=31|issue=4|year=1977|pages=292|issn=0003-6951|doi=10.1063/1.89674|bibcode = 1977ApPhL..31..292S }}</ref> (see [[Staebler–Wronski effect]]). Other materials that were reported to exhibit negative photoconductivity include [[Zinc oxide|ZnO nanowires]],<ref name=":0">{{Cite journal |last1=Javadi |first1=Mohammad |last2=Abdi |first2=Yaser |date=2018-07-30 |title=Frequency-driven bulk-to-surface transition of conductivity in ZnO nanowires |url=https://aip.scitation.org/doi/abs/10.1063/1.5039474 |journal=Applied Physics Letters |volume=113 |issue=5 |pages=051603 |doi=10.1063/1.5039474 |bibcode=2018ApPhL.113e1603J |issn=0003-6951}}</ref> [[molybdenum disulfide]],<ref name="Serpi1992">{{cite journal|last1=Serpi|first1=A.|title=Negative Photoconductivity in MoS2|journal=Physica Status Solidi A|volume=133|issue=2|year=1992|pages=K73–K77|issn=0031-8965|doi=10.1002/pssa.2211330248|bibcode = 1992PSSAR.133...73S }}</ref> [[graphene]],<ref name="HeymanStein2015">{{cite journal|last1=Heyman|first1=J. N.|last2=Stein|first2=J. D.|last3=Kaminski|first3=Z. S.|last4=Banman|first4=A. R.|last5=Massari|first5=A. M.|last6=Robinson|first6=J. T.|title=Carrier heating and negative photoconductivity in graphene|journal=Journal of Applied Physics|volume=117|issue=1|year=2015|pages=015101|issn=0021-8979|doi=10.1063/1.4905192|arxiv = 1410.7495 |bibcode = 2015JAP...117a5101H |s2cid=118531249 }}</ref> [[indium arsenide]] [[nanowire]]s,<ref>{{Cite journal|last1=Alexander-Webber|first1=Jack A.|last2=Groschner|first2=Catherine K.|last3=Sagade|first3=Abhay A.|last4=Tainter|first4=Gregory|last5=Gonzalez-Zalba|first5=M. Fernando|last6=Di Pietro|first6=Riccardo|last7=Wong-Leung|first7=Jennifer|last8=Tan|first8=H. Hoe|last9=Jagadish|first9=Chennupati|date=2017-12-11|title=Engineering the Photoresponse of InAs Nanowires|journal=ACS Applied Materials & Interfaces|language=EN|volume=9|issue=50|pages=43993–44000|doi=10.1021/acsami.7b14415|pmid=29171260|issn=1944-8244|doi-access=free|hdl=1885/237356|hdl-access=free}}</ref> decorated carbon nanotubes,<ref>{{Cite journal|last1=Jiménez-Marín|first1=E.|last2=Villalpando|first2=I.|last3=Trejo-Valdez|first3=M.|last4=Cervantes-Sodi|first4=F.|last5=Vargas-García|first5=J. R.|last6=Torres-Torres|first6=C.|date=2017-06-01|title=Coexistence of positive and negative photoconductivity in nickel oxide decorated multiwall carbon nanotubes|url=https://www.sciencedirect.com/science/article/pii/S0921510717300569|journal=Materials Science and Engineering: B|language=en|volume=220|pages=22–29|doi=10.1016/j.mseb.2017.03.004|issn=0921-5107}}</ref> and metal [[nanoparticle]]s.<ref name="NakanishiBishop2009">{{cite journal|last1=Nakanishi|first1=Hideyuki|last2=Bishop|first2=Kyle J. M.|last3=Kowalczyk|first3=Bartlomiej|last4=Nitzan|first4=Abraham|last5=Weiss|first5=Emily A.|last6=Tretiakov|first6=Konstantin V.|last7=Apodaca|first7=Mario M.|last8=Klajn|first8=Rafal|last9=Stoddart|first9=J. Fraser|last10=Grzybowski|first10=Bartosz A.|title=Photoconductance and inverse photoconductance in films of functionalized metal nanoparticles|journal=Nature|volume=460|issue=7253|year=2009|pages=371–375|issn=0028-0836|doi=10.1038/nature08131|bibcode = 2009Natur.460..371N|pmid=19606145|s2cid=4425298 }}</ref>

Under an applied AC voltage and upon UV illumination, [[Zinc oxide|ZnO]] [[Nanowire|nanowires]] exhibit a continuous transition from positive to negative photoconductivity as a function of the AC frequency.<ref name=":0" /> ZnO nanowires also display a frequency-driven [[metal-insulator transition]] at room temperature. The responsible mechanism for both transitions has been attributed to a competition between bulk conduction and surface conduction.<ref name=":0" /> The frequency-driven bulk-to-surface transition of conductivity is expected to be a generic character of semiconductor nanostructures with the large [[Surface-area-to-volume ratio|surface-to-volume ratio]].