An inverse problem in film/substrate indentation:extracting both the Young's modulus and thickness of films

In an indentation test,the effective Young's modulus of a film/substrate bilayer heterostructure varies with the indentation depth,a phenomenon known as the substrate effect.In previous studies investigating this,only the Young's modulus of the film was unknown.Once the effective Young's modulus of a film/substrate structure is determined at a given contact depth,the Young's modulus of the film can be uniquely determined,i.e.,there is a one-to-one relation between the Young's modulus of the film and the film/substrate effective Young's modulus.However,at times it is extremely challenging or even impossible to measure the film thickness.Furthermore,the precise definition of the layer/film thickness for a two-dimensional material can be problematic.In the current study,therefore,the thickness of the film and its Young's modulus are treated as two unknowns that must be determined.Unlike the case with one unknown,there are infinite combinations of film thickness and Young's modulus which can yield the same effective Young's modulus for the film/substrate.An inverse problem is formulated and solved to extract the Young's modulus and thickness of the film from the indentation depth-load curve.The accuracy and robustness of the inverse problem-solving method are also demonstrated.

Improving the band alignment at PtSe_(2)grain boundaries with selective adsorption of TCNQ

Grain boundaries in two-dimensional(2D)semiconductors generally induce distorted band alignment and interfacial charge,which impair their electronic properties for device applications.Here,we report the improvement of band alignment at the grain boundaries of PtSe_(2),a 2D semiconductor,with selective adsorption of a presentative organic acceptor,tetracyanoquinodimethane(TCNQ).TCNQ molecules show selective adsorption at the PtSe_(2)grain boundary with strong interfacial charge.The adsorption of TCNQ distinctly improves the band alignment at the PtSe_(2)grain boundaries.With the charge transfer between the grain boundary and TCNQ,the local charge is inhibited,and the band bending at the grain boundary is suppressed,as revealed by the scanning tunneling microscopy and spectroscopy(STM/S)results.Our finding provides an effective method for the advancement of the band alignment at the grain boundary by functional molecules,improving the electronic properties of 2D semiconductors for their future applications.

Band alignment and interlayer hybridization in monolayer organic/WSe_(2) heterojunction

Semiconducting heterojunctions(HJs),comprised of atomically thin transition metal dichalcogenides(TMDs),have shown great potentials in electronic and optoelectronic applications.Organic/TMD hybrid bilayers hold enhanced pumping efficiency of interfacial excitons,tunable electronic structures and optical properties,and other superior advantages to these inorganic HJs.Here,we report a direct probe of the interfacial electronic structures of a crystalline monolayer(ML)perylene-3,4,9,10-tetracarboxylic-dianhydride(PTCDA)/ML-WSe_(2) HJ using scanning tunneling microscopy,photoluminescence,and first-principle calculations.Strong PTCDAAA/Se_(2) interfacial interactions lead to appreciable hybridization of the WSe_(2) conduction band with PTCDA unoccupied states,accompanying with a significant amount of PTCDA-to-WSe_(2) charge transfer(by 0.06 e/PTCDA).A type-ll band alignment was directly determined with a valence band offset of-1.69 eV,and an apparent conduction band offset of-1.57 eV.Moreover,we found that the local stacking geometry at the HJ interface differentiates the hybridized interfacial states.