Facet-Dependent Electrical, Photocatalytic, and Optical Properties of Semiconductor Crystals and Their Implications for Applications

Abstract

Recent studies on the electrical conductivity and photocatalytic activity properties of semiconductor nanocrystals such as CuO, AgO, TiO, PbS, and AgPO exposing well-defined surfaces have revealed strong facet effects. For example, the electrical conductivity of CuO crystals can vary from highly conductive to nonconductive, and they can be highly photocatalytically active or inactive depending on the exposed faces. The crystal surfaces can even tune their light absorption wavelengths. Our understanding is that the emergence of these unusual phenomena can be explained in terms of the presence of an ultrathin surface layer having different band structures and degrees of band bending for different surfaces, which affects charge transport and photons into and out of the crystals. This review uses primarily results from our research on this frontier area of semiconductor properties to illustrate the existence of semiconductor facet effects. A simple adjustment to normal semiconductor band diagram allows good understanding of the observed phenomena. Recognizing that facet-dependent behaviors are intrinsic semiconductor properties, we should pay attention to their influence in the explanation of the measured photocatalytic properties, and consider ways to enhance photocatalytic efficiency or design electrical components utilizing the facet effects. There should be many opportunities to advance applications of semiconductor nanocrystals and nanostructures with continued research on the facet-dependent properties of various semiconductor materials.

Density Functional Theory Calculations Revealing Metal‐like Band Structures for Ultrathin Germanium (111) and (211) Surface Layers

“…These subtle differences can explain why high‐resolution electron microscopy and the current surface‐sensitive analytical techniques are unable to detect the existence of this surface layer that results in various facet‐dependent effects of semiconductor crystals. It is expected that such subtle bond deviations and frontier‐electron energy distributions are also present on the various surfaces of other semiconductor crystals, because facet‐dependent phenomena are broadly observable in many semiconductor materials …”

“…It is expectedt hat such subtle bond deviations and frontier-electron energy distributions are also present on the various surfaces of other semiconductor crystals, because facet-dependent phenomena are broadly observable in many semiconductor materials. [9] An implication of this work is that if Fin field-effect transistors (FinFETs) are fabricatedo nag ermanium wafer with transistor side walls exposing Ge(111)s urfaces after the chemical etchingp rocess, the fin width mayn ot be thinner than 3nm, as the thin surface layer that shows am etal-like band structure is approximately 1.5 nm. [20] Af in width less than 3nmc an lose its semiconducting property,m aking the transistor inoperable.…”

“…This model is also very useful to explain widely observed facet-dependent photocatalytic properties of many semiconductor materials and heterostructures, in which photocatalytic activities can vary from highly active to completely suppressed depending on the exposed or contacting facets as seen in Cu 2 Oc rystals and Cu 2 O-ZnOh eterostructures. [9][10][11][12][13][14][15] As the ultrathin layer has dissimilar band structures for different crystal faces it meanst hat light of somewhat differentw avelengths is absorbed by particles of variouss hapes, accounting for the observedf acet-dependent opticalp roperties of Cu 2 Oa nd other semiconductor crystals. [7,11,[16][17][18] To verify that the facet-dependent electrical-conductivity phenomenon is broadly observable in semiconductors, electrical-conductivity measurementsh ave been made on four different faces of Si wafers, which revealed that the (111)a nd (112) surfaces are highly conductive.…”

Density Functional Theory Calculations Revealing Metal‐like Band Structures and Work Function Variation for Ultrathin Gallium Arsenide (111) Surface Layers

“…[8] Furthermore, Cu 2 O, Ag 2 O, and Ag 3 PO 4 crystals, as well as Cu 2 Obased semiconductor heterojunctions, also present facet-dependento ri nterfacial plane-relatedp hotocatalytic properties from high photocatalytic efficiency to total deactivation. [8][9][10] The emergence of these semiconductor facet effects should originate from the presence of an ultrathin surface layer with dissimilar band structures for different crystal planesa sr evealed through DFT calculations, such that charge carriers encounter different barrierh eights across ap articular crystal surface. [1][2][3][10][11][12] This ultrathin surface layer should also be respon-sible fort he observation of opticalf acet effects demonstrated in polyhedralC u 2 Oa nd Ag 3 PO 4 nanocrystals, suggesting light absorption of semiconductor crystals actually has bulk and surface components.…”

Ultrafine MoP Nanoparticles Well Embedded in Carbon Nanosheets as Electrocatalyst with High Active Site Density for Hydrogen Evolution

“…Mo 2 C quantum dots with average diameter of 2 nm were also successfully obtained having a large double layer capacitance . In addition, many transition metal compounds shows strong facet‐dependent effect which has a great influence on the catalytic performance . Ultrafine NPs are in favor of exposing more available facets for HER.…”