Femtosecond Time-Resolved Spectroscopic Photoemission Electron Microscopy for Probing Ultrafast Carrier Dynamics in Heterojunctions

The fast developing semiconductor industry is pushing to shrink and speed up transistors. This trend requires us to understand carrier dynamics in semiconductor heterojunctions with both high spatial and temporal resolutions. Recently, we have successfully set up a timeresolved photoemission electron microscopy (TR-PEEM), which integrates the spectroscopic technique to measure electron densities at specific energy levels in space. This instrument provides us an unprecedented access to the evolution of electrons in terms of spatial location, time resolution, and energy, representing a new type of 4D spectro-microscopy. Here in this work, we present measurements of semiconductor performance with a time resolution of 184 fs, electron kinetic energy resolution of 150 meV, and spatial resolution of about 150 nm or better. We obtained time-resolved micro-area photoelectron spectra and energy-resolved TR-PEEM images on the Pb island on Si(111). These experimental results suggest that this instrument has the potential to be a powerful tool for investigating the carrier dynamics in various heterojunctions, which will deepen our understanding of semiconductor properties in the submicron/nanometer spatial scales and ultrafast time scales.

Assessment of circulating tumor DNA in cerebrospinal fluid by whole exome sequencing to detect genomic alterations of glioblastoma

Background:Cerebrospinal fluid(CSF)has been demonstrated as a better source of circulating tumor DNA(ctDNA)than plasma for brain tumors.However,it is unclear whether whole exome sequencing(WES)is qualified for detection of ctDNA in CSF.The aim of this study was to determine if assessment of ctDNA in CSF by WES is a feasible approach to detect genomic alterations of glioblastoma.Methods:CSFs of ten glioblastoma patients were collected pre-operatively at the Department of Neurosurgery,Sun Yat-sen University Cancer Center.ctDNA in CSF and genome DNA in the resected tumor were extracted and subjected to WES.The identified glioblastoma-associated mutations from ctDNA in CSF and genome DNA in the resected tumor were compared.Results:Due to the ctDNA in CSF was unqualified for exome sequencing for one patient,nine patients were included into the final analysis.More glioblastoma-associated mutations tended to be detected in CSF compared with the corresponding tumor tissue samples(3.56±0.75 vs.2.22±0.32,P=0.097),while the statistical significance was limited by the small sample size.The average mutation frequencies were similar in CSF and tumor tissue samples(74.1%±6.0%vs.73.8%±6.0%,P=0.924).The R132H mutation of isocitrate dehydrogenase 1 and the G34V mutation of H3 histone,family 3A(H3F3A)which had been reported in the pathological diagnoses were also detected from ctDNA in CSF by WES.Patients who received temozolomide chemotherapy previously or those whose tumor involved subventricular zone tended to harbor more mutations in their CSF.Conclusion:Assessment of ctDNA in CSF by WES is a feasible approach to detect genomic alterations of glioblastoma,which may provide useful information for the decision of treatment strategy.

An efficient route to prepare suspended monolayer for feasible optical and electronic characterizations of two-dimensional materials

Two-dimensional(2D)materials are highly sensitive to substrates,interfaces,and the surrounding environments.Suspended 2D materials are free from substrate-induced effects,thus an ideal approach to study their intrinsic properties.However,it is very challenging to prepare large-area suspended 2D materials with high efficiency.Here we report a universal method,based on pretreatments of densely patterned hole array substrates with either oxygenplasma or gold film deposition,to prepare large-area suspended mono-and few-layer 2D materials.Multiple structural,optical,and electrical characterization tools were used to fully evaluate the improved performance of various suspended 2D layers.Some of these observations reported in this study are:(1)Observation of a new Raman low frequency mode for the suspended MoS_(2);(2)Significantly stronger photoluminescence(PL)and second harmonic generation(SHG)signals of suspended WSe_(2),which enables the study of new optical transition processes;(3)The low energy electron diffraction pattern on suspended MoS_(2) also exhibits much sharper spots than that on the supported area;and(4)The mobility of suspended graphene device approaches 300000 cm^(2) V^(-1) s^(-1),which is desirable to explore the intrinsic properties of graphene.This work provides an innovative and efficient route for fabricating suspended 2D materials,and we expect that it can be broadly used for studying intrinsic properties of 2D materials and in applications of hybrid active nanophotonic and electronic devices.