Christopher T. Chen

Christopher T. Chen
Lawrence Berkeley National Laboratory | LBL · Molecular Foundry

Doctor of Philosophy

About

39
Publications
6,029
Reads
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816
Citations
Citations since 2017
23 Research Items
737 Citations
2017201820192020202120222023050100150200
2017201820192020202120222023050100150200
2017201820192020202120222023050100150200
2017201820192020202120222023050100150200
Additional affiliations
December 2015 - present
Lawrence Berkeley National Laboratory
Position
  • PostDoc Position
September 2015 - November 2015
California Institute of Technology
Position
  • PostDoc Position
January 2010 - August 2015
California Institute of Technology
Position
  • Research Assistant

Publications

Publications (39)
Preprint
Carrier and phonon dynamics in a multilayer WSe2 film are captured by extreme ultraviolet (XUV) transient absorption (TA) spectroscopy at the W N6,7, W O2,3, and Se M4,5 edges (30-60 eV). After the broadband optical pump pulse, the XUV probe directly reports on occupations of optically excited holes and phonon-induced band renormalizations. By comp...
Article
Few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy, performed with optical 500–1000-nm supercontinuum and broadband XUV pulses (30–50 eV), simultaneously probes dynamics of photoexcited carriers in WS2 at the W O3 edge (37–45 eV) and carrier-induced modifications of core-exciton absorption at the WN6,7 edge (32–37 eV). Acce...
Article
Full-text available
Surface plasmons have found a wide range of applications in plasmonic and nanophotonic devices. The combination of plasmonics with three-dimensional photonic crystals has enormous potential for the efficient localization of light in high surface area photoelectrodes. However, the metals traditionally used for plasmonics are difficult to form into t...
Preprint
Attosecond transient absorption spectroscopy, performed with optical 500-1000 nm supercontinuum and broadband extreme ultraviolet (XUV) pulses (30-50 eV), simultaneously probes dynamics of photoexcited carriers in WS$_{2}$ at the W O$_3$ edge (37-45 eV) and carrier-induced modifications of core-exciton absorption at the W N$_{6,7}$ edge (32-37 eV)....
Article
Full-text available
Atomically thin polycrystalline transition-metal dichalcogenides (TMDs) are relevant to both fundamental science investigation and applications. TMD thin-films present uniquely difficult challenges to effective nanoscale crystalline characterization. Here we present a method to quickly characterize the nanocrystalline grain structure and texture of...
Article
Two-dimensional (2D) excitons arise from electron-hole confinement along one spatial dimension. Such excitations are often described in terms of Frenkel or Wannier limits according to the degree of exciton spatial localization and the surrounding dielectric environment. In hybrid material systems, such as the 2D perovskites, the complex underlying...
Preprint
Full-text available
Control of impurity concentrations in semiconducting materials is essential to device technology. Because of their intrinsic confinement, the properties of two-dimensional semiconductors such as transition metal dichalcogenides (TMDs) are more sensitive to defects than traditional bulk materials. The technological adoption of TMDs is dependent on t...
Preprint
Atomically thin polycrystalline transition-metal dichalcogenides (TMDs) are relevant to both fundamental science investigation and applications. TMD thin-films present uniquely difficult challenges to effective nanoscale crystalline characterization. Here we present a method to quickly characterize the nanocrystalline grain structure and texture of...
Article
Light emitters in wide-band-gap semiconductors are of great fundamental interest and have potential as optically addressable qubits. Here we describe a unique color center in high-quality hexagonal boron nitride (h-BN) with a sharp emission line at 435 nm. The emitters are activated and deactivated by electron beam irradiation and have spectral and...
Article
Structural defects in 2D materials offer an effective way to engineer new material functionalities beyond conventional doping. We report on the direct experimental correlation of the atomic and electronic structure of a sulfur vacancy in monolayer WS2 by a combination of CO-tip noncontact atomic force microscopy and scanning tunneling microscopy. S...
Article
Control of impurity concentrations in semiconducting materials is essential to device technology. Because of their intrinsic confinement, the properties of two-dimensional semiconductors such as transition metal dichalcogenides (TMDs) are more sensitive to defects than traditional bulk materials. The technological adoption of TMDs is dependent on t...
Article
Full-text available
Chalcogen vacancies are generally considered to be the most common point defects in transition metal dichalcogenide (TMD) semiconductors because of their low formation energy in vacuum and their frequent observation in transmission electron microscopy studies. Consequently, unexpected optical, transport, and catalytic properties in 2D-TMDs have bee...
Preprint
Light emitters in wide band gap semiconductors are of great fundamental interest and have potential as optically addressable qubits. Here we describe the discovery of a new quantum emitter in high-quality hexagonal boron nitride (h-BN) with a sharp emission line at 435 nm. The emitters are activated and deactivated by electron beam irradiation and...
Article
Full-text available
Materials for nanophotonic devices ideally combine ease of deposition, very high refractive index, and facile pattern formation through lithographic templating and/or etching. In this work, we present a scalable method for producing high refractive index WS2 layers by chemical conversion of WO3 synthesized via atomic layer deposition (ALD). These c...
Article
Two-dimensional materials with engineered composition and structure will provide designer materials beyond conventional semiconductors. However, the potentials of defect engineering remain largely untapped, because it hinges on a precise understanding of electronic structure and excitonic properties, which are not yet predictable by theory alone. H...
Preprint
Full-text available
Chalcogen vacancies are considered to be the most abundant point defects in two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors, and predicted to result in deep in-gap states (IGS). As a result, important features in the optical response of 2D-TMDs have typically been attributed to chalcogen vacancies, with indirect support fr...
Preprint
Full-text available
Structural defects in 2D materials offer an effective way to engineer new material functionalities beyond conventional doping in semiconductors. Specifically, deep in-gap defect states of chalcogen vacancies have been associated with intriguing phenomena in monolayer transition metal dichalcogenides (TMDs). Here, we report the direct experimental c...
Conference Paper
Full-text available
We report on an study of the GaP/Si interface for application in silicon heterojunction solar cells. We analyzed the band alignment using X-ray photoelectron spectroscopy (XPS) and cross-sectional Kelvin probe force microscopy (x-KPFM). Our measurements show a high conduction band offset (0.9 eV) leading to a barrier in electron extraction which we...
Article
Full-text available
We have investigated the GaP/Si heterojunction interface for application in silicon heterojunction solar cells. We performed X-ray photoelectron spectroscopy (XPS) on thin layers of GaP grown on Si by metal organic chemical vapor deposition and molecular beam epitaxy. The conduction band offset was determined to be 0.9 ± 0.2 eV, which is significan...
Article
Full-text available
Van der Waals epitaxy enables the integration of 2D transition metal dichalcogenides with other layered materials to form heterostructures with atomically sharp interfaces. However, the ability to fully utilize and understand these materials using surface science techniques such as angle resolved photoemission spectroscopy (ARPES) and scanning tunn...
Article
Full-text available
Due to its high refractive index and low absorption coefficient, gallium phosphide is an ideal material for photonic structures targeted at the visible wavelengths. However, these properties are only realized with high quality epitaxial growth, which limits substrate choice and thus possible photonic applications. In this work, we report the fabric...
Article
Finding solutions to improve the performance of semiconductor light absorbers and catalyst materials remains an outstanding issue that prevents the realization of solar fuel generators. Nanostructuring approaches of photoelectrocatalytic materials have the potential to reduce bulk recombination and improve electron–hole pair separation in semicondu...
Article
2D transition metal dichalcogenides (TMDs) are commonly grown by chemical vapor deposition using transition metal oxides as solid precursors. Despite the widespread use of this technique, challenges in reproducibility, coverage, and material quality are pervasive, suggestive of unknown and uncontrolled process parameters. In this communication, we...
Article
Full-text available
We report ordered, high aspect ratio, tapered Si microwire arrays that exhibit an extremely-low angular (0o to 50o) and spectrally averaged reflectivity of <1% of the incident 400 nm - 1100 nm illumination. After isolating the microwires from the substrate with a polymer infill and peel off process, the arrays were found to absorb 89.1% of angular...
Article
Selected area Cu-catalyzed vapor-liquid-solid growth of SiGe microwires is achieved using chlorosilane and chlorogermane precursors. The composition can be tuned up to 12% Ge with a simultaneous decrease in the growth rate from 7 to 1 μm min-1. Significant changes to the morphology were observed, including tapering and faceting on the sidewalls and...
Conference Paper
The world record efficiency and open circuit voltage for crystalline silicon solar cells are held by a-Si/Si heterojunction devices. While a-Si provides excellent surface passivation, these heterojunction devices are limited by non-ideal optical and electronic properties. Gallium phosphide is a candidate material for replacing a-Si in a heterojunct...
Article
A transparent, flexible contact is developed using Ni nanoparticles and Ag nanowires and demonstrated on free-standing, polymer embedded, Si microwire solar cells. Contact yields of over 99% and a series resistance of 14 Ω cm(2) are demonstrated.
Article
Wire arrays have demonstrated promising photovoltaic performance as single junction solar cells and are well suited to defect mitigation in heteroepitaxy. These attributes can combine in tandem wire array solar cells, potentially leading to high efficiencies. Here, we demonstrate initial growths of GaAs on Si0.9Ge0.1 structures and investigate III-...
Conference Paper
Tandem Ga1-xInxP/Si microwire array solar cells are a route towards a high efficiency, low cost, flexible, wafer-free solar technology. Coupled full-field optical and device physics simulations of a Ga0.51In0.49P/Si wire array tandem are used to predict device performance. A 500 nm thick, highly doped “buffer” layer between the bottom cell and tunn...
Article
Gallium phosphide exhibits a short diffusion length relative to its optical absorption length, and is thus a candidate for use in wire array geometries that allow light absorption to be decoupled from minority carrier collection. Herein is reported the photoanodic performance of heteroepitaxially grown gallium phosphide on planar and microwire-arra...
Article
Silicon microwire arrays have recently demonstrated their potential for low-cost, high-efficiency photovoltaics and photoelectrochemical fuel generation. A remaining challenge to making this technology commercially viable is scaling up of microwire-array growth. We discuss here a technique for vapor–liquid–solid growth of microwire arrays on the sc...
Article
While significant progress has been achieved in the fabrication and performance of transparent electronic devices, substantially less research effort has been devoted to transparent interconnects, despite their critical importance for transparent integrated circuitry. Here, we exploit the crystal disorder induced by Ar+ ion bombardment to achieve e...
Article
Silicon microwire arrays have recently demonstrated their potential for low cost, high efficiency photovoltaics. These high aspect ratio, radial junction wire arrays allow for the absorption of nearly all the incident sunlight while enabling efficient carrier extraction in the radial direction. One of the remaining challenges to make this technolog...
Conference Paper
Microwire solar cells have demonstrated promising optical and photovoltaic performance in arrays of single junction Si wires. Seeking higher efficiencies, we have numerically investigated III-V on Si1-xGex architectures as candidates for tandem microwire photovoltaics via optical and electronic transport modeling. Optical modeling indicates that li...
Conference Paper
Silicon microwire arrays have recently demonstrated their potential for low cost, high efficiency photovoltaics. These high aspect ratio, radial junction wire arrays allow for the absorption of nearly all the incident sunlight while enabling efficient carrier extraction in the radial direction. One of the remaining challenges to make this technolog...
Article
Realizing optically transparent functional circuitry continues to fuel scientific and technological interest in transparent conducting oxides (TCOs). However, precise means for creating transparent interconnects for device-to-device integration has remained elusive. Here we report on the chemical, microstructural, and electronic properties of trans...
Article
An effective method for the spatially-controlled writing of embedded, optically transparent, electrically conducting oxide nanowires, through focused ion beam (FIB) implantation into highly resistive transparent metal oxide thin films, was reported. Highly resistive In2O3 thin films were deposited on glass substrates with horizontal dual gun ion as...
Article
A conductive atomic force microscopy tip probes an embedded, optically transparent, electrically conducting oxide nanowire that was patterned on an indium oxide substrate using focused ion beam implantation. The nanowire is 160 nm wide, 7 nm deep, and theoretically limitless in length, connectivity, and shape. Nanowires of this type have potential...

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