Nanoscale Surface Photovoltage Mapping of 2D Materials and Heterostructures by Illuminated Kelvin Probe Force Microscopy

Abstract

Nanomaterials are interesting for a variety of applications, such as optoelectronics and photovoltaics. However, they often have spatial heterogeneity, i.e. composition change or physical change in the topography or structure, which can lead to varying properties that would influence their applications. New techniques must be developed to understand and correlate spatial heterogeneity with changes in electronic properties. Here we highlight the technique of surface photovoltage-Kelvin probe force microscopy (SPV-KFM), which is a modified version of noncontact atomic force microscopy capable of imaging not only the topography and surface potential, but also the surface photovoltage on the nanoscale. We demonstrate its utility in probing monolayer WSe 2-MoS 2 lateral heterostructures, which form an ultrathin p-n junction promising for photovoltaic and optoelectronic applications. We show surface photovoltage maps highlighting the different photoresponse of the two material regions as a result of the effective charge separation across this junction. Additionally, we study the variations between different

Charge Separation in Epitaxial SnS/MoS2 Vertical Heterojunctions Grown by Low-Temperature Pulsed MOCVD

“…Kelvin probe force microscopy (KPFM) has previously been used to measure the surface photovoltage of van der Waals heterojunctions; , here we use KPFM to detect vertical charge carrier separation resulting from the formation of an electronic heterojunction between SnS and MoS 2 . KPFM uses a conductive AFM tip to measure the topography and contact potential difference (CPD) of the sample; the work function is then determined by comparison with the CPD of a reference material, in this case, highly oriented pyrolytic graphite.…”

Controllable Water Vapor Assisted Chemical Vapor Transport Synthesis of WS2–MoS2 Heterostructure

“…[66][67][68][69] Note the boundaries between WS2 and MoS2 also showed brighter response in both PL and Raman. This could be related to potential charge transfer at the heterojunction of monolayer WS2 and MoS2, 70 which needs to be further invesigated. Figure 4g and 4h also consists of 3 layers of WS2 and MoS2, which was confirmed by AFM as shown in Figure S7.…”

“…[66][67][68][69] Note the boundaries between WS2 and MoS2 also showed brighter response in both PL and Raman. This could be related to potential charge transfer at the heterojunction of monolayer WS2 and MoS2, 70 which needs to be further invesigated. This few layer WS2-MoS2 lateral heterostructure showed the same difference in optical contrast seen in monolayer heterostructures, however the three-fold "radioactive symbol" is much less obvious.…”

SnS2 Nanosheets/H‐TiO2 Nanotube Arrays as a Type II Heterojunctioned Photoanode for Photoelectrochemical Water Splitting

“…[32] With many influence factorsi na mbient conditions, such as surface adsorbate or tip conditions, the accurate work-function values of thesem aterials were not calculated. [33] The CPD of the SnS 2 nanosheet was highert han that of TiO 2 ,i mplying that the work function of TiO 2 was larger than that of SnS 2 .T he values for the valence-band edge and the band gap were À7.4 and 3.33 eV for TiO 2 ,r espectively, [34] and À6.44 and 2.26 eV for SnS 2 . [35] The conduction-band edge of SnS 2 was more negative (À4.18 eV) than that of TiO 2 (À4.07 eV).…”

“…(standard cubic centimeter per minute) argon.A SnS 2 nanosheet tightly bonded with TiO 2 NTsw as chosen for KPFM measurements. [33] The CPD of the SnS 2 nanosheet was highert han that of TiO 2 ,i mplying that the work function of TiO 2 was larger than that of SnS 2 .T he values for the valence-band edge and the band gap were À7.4 and 3.33 eV for TiO 2 ,r espectively, [34] and À6.44 and 2.26 eV for SnS 2 . The average CPD between SnS 2 and H-TiO 2 was approximately 40 mV for SnS 2 /H-TiO 2 ,m uch smaller than that between SnS 2 and TiO 2 ( % 75 mV) based on the KPFM images in the dark state (Figure 4b www.chemsuschem.org surfacet op articipate in the oxidation process for at ypicalntype semiconductor photoanode and the electrons migrated to the substrate and were transferred to the cathode for the reduction reaction.…”