arXiv subjects

Stephan Hofmann

Publications and source records attributed to Stephan Hofmann.

At least 37 records · Page 2Linked to original sources

arXiv2021

Formation of moiré interlayer excitons in space and time

Moiré superlattices in atomically thin van-der-Waals heterostructures hold great promise for an extended control of electronic and valleytronic lifetimes, the confinement of excitons in artificial moiré lattices, and the formation of novel exotic quantum phases. Such moiré-induced emergent phenomena are particularly strong for interlayer excitons, where the hole and the electron are localized in different layers of the heterostructure. In order to exploit the full potential of correlated moiré and exciton physics, a thorough understanding of the ultrafast interlayer exciton formation process and the real-space wavefunction confinement in the moiré potential is indispensable. However, direct experimental access to these parameters is limited since most excitonic quasiparticles are optically dark. Here we show that femtosecond photoemission momentum microscopy provides quantitative access to these key properties of the moiré interlayer excitons. We find that interlayer excitons are dominantly formed on the sub-50~fs timescale via interlayer tunneling at the K valleys of the Brillouin zones. In addition, we directly measure energy-momentum fingerprints of the moiré interlayer excitons by mapping their spectral signatures within the mini Brillouin zone that is built up by the twisted heterostructure. From these momentum-fingerprints, we gain quantitative access to the modulation of the exciton wavefunction within the moiré potential in real-space. Our work provides the first direct access to the interlayer moiré exciton formation dynamics in space and time and reveals new opportunities to study correlated moiré and exciton physics for the future realization of exotic quantum phases of matter.

cond-mat.mes-hall↗

arXiv2021

Bridging the Band Gap: What Device Physicists Need to Know About Machine Learning

This article surveys the landscape of semiconductor materials and devices research for the acceleration of machine learning (ML) algorithms. We observe a disconnect between the semiconductor and device physics and engineering communities, and the digital logic and computer hardware architecture communities. The article first provides an overview of the principles of computational complexity and fundamental physical limits to computing and their relation to physical systems. The article then provides an introduction to ML by presenting three key components of ML systems: representation, evaluation, and optimisation. The article then discusses and provides examples of the application of emerging technologies from the demiconductor and device physics domains as solutions to computational problems, alongside a brief overview of emerging devices for computing applications. The article then reviews the landscape of ML accelerators, comparing fixed-function and reprogrammable digital logic with novel devices such as memristors, resistive memories, magnetic memories, and probabilistic bits. We observe broadly lower performance of ML accelerators based on novel devices and materials when compared to those based on digital complimentary metal-oxide semiconductor (CMOS) technology, particularly in the MNIST optical character recognition task, a common ML benchmark, and also highlight the lack of a trend of progress in approaches based on novel materials and devices. Lastly, the article proposes figures of merit for meaningful evaluation and comparison of different ML implementations in the hope of fostering a dialogue between the materials science, device physics, digital logic, and computer architecture communities by providing a common frame of reference for their work.

cs.ET↗

arXiv2021

Active terahertz modulator and slow light metamaterial devices with hybrid graphene-superconductor photonic integrated circuits

Metamaterial photonic integrated circuits with arrays of hybrid graphene-superconductor coupled split-ring resonators (SRR) capable of modulating and slowing down terahertz (THz) light are introduced and proposed. The hybrid device optical responses, such as electromagnetic induced transparency (EIT) and group delay, can be modulated in several ways. First, it is modulated electrically by changing the conductivity and carrier concentrations in graphene. Alternatively, the optical response can be modified by acting on the device temperature sensitivity, by switching Nb from a lossy normal phase to a low-loss quantum mechanical phase below the transition temperature (Tc) of Nb. Maximum modulation depths of 57.3 % and 97.61 % are achieved for EIT and group delay at the THz transmission window, respectively. A comparison is carried out between the Nb-graphene-Nb coupled SRR-based devices with those of Au-graphene-Au SRRs and a significant enhancement of the THz transmission, group delay, and EIT responses are observed when Nb is in the quantum mechanical phase. Such hybrid devices with their reasonably large and tunable slow light bandwidth pave the way for the realization of active optoelectronic modulators, filters, phase shifters, and slow light devices for applications in chip-scale quantum communication and quantum processing.

physics.optics↗

arXiv2021

Giant Photoluminescence Enhancement in MoSe$_{2}$ monolayers treated with Oleic Acid Ligands

The inherently low photoluminescence (PL) yields in as prepared transition metal dichalcogenide (TMD) monolayers are broadly accepted to be the result of atomic vacancies (i.e. defects) and uncontrolled doping, which give rise to non-radiative exciton decay pathways. To date, a number of chemical passivation schemes have been successfully developed to improve PL in sulphur based TMDs i.e. molybdenum disulphide (MoS2) and tungsten disulphide (WS2) monolayers. Reports on solution based chemical passivation schemes for improving PL yields in selenium (Se) based TMDs are lacking I comparison, with only one known study that uses hydrobromic acid vapour to improve PL in chemical vapour deposited (CVD) Molybdenum diselenide (MoSe2). Here, we demonstrate that treatment with oleic acid (OA) provides a simple wet chemical passivation method for monolayer MoSe2, enhancing PL yield by an average of 58 fold, while also enhancing spectral uniformity across the material and reducing emission linewidth. Excitation intensity dependent PL reveals trap-free PL dynamics dominated by neutral exciton recombination. Time-resolved PL (TRPL) studies reveal significantly increased PL lifetimes, with pump intensity dependent TRPL measurements also confirming trap free PL dynamics in OA treated MoSe2. Field effect transistors show reduced charge trap density and improved on-off ratios after treatment with OA. These results indicate defect passivation by OA, which we hypothesise act as ligands, passivating chalcogen defects through oleate coordination to Mo dangling bonds.

cond-mat.mtrl-sci↗

arXiv2020

Oxidising and carburising catalyst conditioning for the controlled growth and transfer of large crystal monolayer hexagonal boron nitride

Hexagonal boron nitride (h-BN) is well-established as a requisite support, encapsulant and barrier for 2D material technologies, but also recently as an active material for applications ranging from hyperbolic metasurfaces to room temperature single-photon sources. Cost-effective, scalable and high quality growth techniques for h-BN layers are critically required. We utilise widely-available iron foils for the catalytic chemical vapour deposition (CVD) of h BN and report on the significant role of bulk dissolved species in h-BN CVD, and specifically, the balance between dissolved oxygen and carbon. A simple pre-growth conditioning step of the iron foils enables us to tailor an error-tolerant scalable CVD process to give exceptionally large h-BN monolayer domains. We also develop a facile method for the improved transfer of as-grown h-BN away from the iron surface by means of the controlled humidity oxidation and subsequent rapid etching of a thin interfacial iron oxide; thus, avoiding the impurities from the bulk of the foil. We demonstrate wafer-scale (2 inch) production and utilise this h-BN as a protective layer for graphene towards integrated (opto) electronic device fabrication.

cond-mat.mtrl-sci↗

arXiv2020

The bright side of defects in MoS$_2$ and WS$_2$ and a generalizable chemical treatment protocol for defect passivation

Structural defects are widely regarded as detrimental to the optoelectronic properties of monolayer transition metal dichalcogenides, leading to concerted efforts to eliminate defects via improved materials growth or post-growth passivation. Here, using steady-state and ultrafast optical spectroscopy, supported by ab initio calculations, we demonstrate that sulfur vacancy defects act as exciton traps. Current chemical treatments do not passivate these sites, leading to decreased mobility and trap-limited photoluminescence. We present a generalizable treatment protocol based on the use of passivating agents such as thiols or sulfides in combination with a Lewis acid to passivate sulfur vacancies in monolayer MoS$_2$ and WS$_2$, increasing photoluminescence up to 275 fold, while maintaining mobilities. Our findings suggest a route for simple and rational defect engineering strategies, where the passivating agent varies the electronic properties, thereby allowing the design of new heterostructures.

cond-mat.mtrl-sci↗

arXiv2019

Resolving the Nucleation Stage in Atomic Layer Deposition of Hafnium Oxide on Graphene

The integration of two-dimensional (2D) materials with functional non-2D materials such as metal oxides is of key importance for many applications, but underlying mechanisms for such non-2D/2D interfacing remain largely elusive at the atomic scale. To address this, we here investigate the nucleation stage in atomic layer deposition (ALD) of the important metal oxide HfO2 on chemical vapor deposited graphene using atomically resolved and element specific scanning transmission electron microscopy (STEM). To avoid any deleterious influence of polymer residues from pre-ALD graphene transfers we employ a substrate-assisted ALD process directly on the as grown graphene still remaining on its Cu growth catalyst support. Thereby we resolve at the atomic scale key factors governing the integration of non-2D metal oxides with 2D materials by ALD: Particular to our substrate-assisted ALD process we find a graphene-layer-dependent catalytic participation of the supporting Cu catalyst in the ALD process. We further confirm at high resolution the role of surface irregularities such as steps between graphene layers on oxide nucleation. Employing the energy transfer from the scanning electron beam to in situ crystallize the initially amorphous ALD HfO2 on graphene, we observe HfO2 crystallization to non-equilibrium HfO2 polymorphs (cubic/tetragonal). Finally our data indicates a critical role of the graphene's atmospheric adventitious carbon contamination on the ALD process whereby this contamination acts as an unintentional seeding layer for metal oxide ALD nucleation on graphene under our conditions. As atmospheric adventitious carbon contamination is hard to avoid in any scalable 2D materials processing, this is a critical factor in ALD recipe development for 2D materials coating. Combined our work highlights several key mechanisms underlying scalable ALD oxide growth on 2D materials.

cond-mat.mtrl-sci↗

arXiv2019

Tunable Klein-like tunneling of high-temperature superconducting pairs into graphene

Superconductivity can be induced in a normal material via the leakage of superconducting pairs of charge carriers from an adjacent superconductor. This so-called proximity effect is markedly influenced by graphene unique electronic structure, both in fundamental and technologically relevant ways. These include an unconventional form of the leakage mechanism the Andreev reflection and the potential of supercurrent modulation through electrical gating. Despite the interest of high-temperature superconductors in that context, realizations have been exclusively based on low-temperature ones. Here we demonstrate gate-tunable, high-temperature superconducting proximity effect in graphene. Notably, gating effects result from the perfect transmission of superconducting pairs across an energy barrier -a form of Klein tunneling, up to now observed only for non-superconducting carriers- and quantum interferences controlled by graphene doping. Interestingly, we find that this type of interferences become dominant without the need of ultra-clean graphene, in stark contrast to the case of low-temperature superconductors. These results pave the way to a new class of tunable, high-temperature Josephson devices based on large-scale graphene.

cond-mat.mes-hall↗

arXiv2019

Wide-field spectral super-resolution mapping of optically active defects in hBN

Point defects can have significant impacts on the mechanical, electronic and optical properties of materials. The development of robust, multidimensional, high-throughput and large-scale characterization techniques of defects is thus crucial, from the establishment of integrated nanophotonic technologies to material growth optimization. Here, we demonstrate the potential of wide-field spectral single-molecule localization microscopy (spectral SMLM) for the determination of ensemble spectral properties, as well as characterization of spatial, spectral and temporal dynamics of single defects in CVD-grown and irradiated exfoliated hexagonal boron-nitride (hBN) materials. We characterize the heterogeneous spectral response of our samples, and identify at least two types of defects in CVD-grown materials, while irradiated exfoliated flakes show predominantly only one type of defect. We analyze the blinking kinetics and spectral emission for each type of defects, and discuss their implications with respect to the observed spectral heterogeneity of our samples. Our study shows the potential of wide-field spectral SMLM techniques in material science and paves the way towards quantitative multidimensional mapping of defect properties.

physics.optics↗

arXiv2019

Non-equilibrium properties of graphene probed by superconducting tunnel spectroscopy

We report on non-equilibrium properties of graphene probed by superconducting tunnel spectroscopy. A hexagonal boron nitride (hBN) tunnel barrier in combination with a superconducting Pb contact is used to extract the local energy distribution function of the quasiparticles in graphene samples in different transport regimes. In the cases where the energy distribution function resembles a Fermi-Dirac distribution, the local electron temperature can directly be accessed. This allows us to study the cooling mechanisms of hot electrons in graphene. In the case of long samples (device length $L$ much larger than the electron-phonon scattering length $l_{e-ph}$), cooling through acoustic phonons is dominant. We find a cross-over from the dirty limit with a power law $~T^3$ at low temperature to the clean limit at higher temperatures with a power law $~T^4$ and a deformation potential of 13.3 eV. For shorter samples, where $L$ is smaller than $l_{e-ph}$ but larger than the electron-electron scattering length $l_{e-e}$, the well-known cooling through electron out-diffusion is found. Interestingly, we find strong indications of an enhanced Lorenz number in graphene. We also find evidence of a non-Fermi-Dirac distribution function, which is a result of non-interacting quasiparticles in very short samples.

cond-mat.mes-hall↗

arXiv2019

Spectrally-Resolved Photodynamics of Individual Emitters in Large-Area Monolayers of Hexagonal-Boron Nitride

Hexagonal boron nitride (h-BN) is a 2D, wide band-gap semiconductor that has recently been shown to display bright room-temperature emission in the visible region, sparking immense interest in the material for use in quantum applications. In this work, we study highly crystalline, single atomic layers of chemical vapour deposition (CVD)-grown hexagonal boron nitride and find predominantly one type of emissive state. Using a multidimensional super-resolution fluorescence microscopy technique we simultaneously measure spatial position, intensity and spectral properties of the emitters, as they are exposed to continuous wave illumination over minutes. As well as low emitter heterogeneity, we observe inhomogeneous broadening of emitter line-widths and power law dependency in fluorescence intermittency, this is in striking similarity to previous work on quantum dots. These results show that high control over h-BN growth and treatment can produce a narrow distribution of emitter type, and that surface interactions heavily influence the photodynamics. Furthermore, we highlight the utility of spectrally-resolved wide-field microscopy in the study of optically-active excitons in atomically thin two-dimensional materials.

physics.optics↗

arXiv2018

Compressive Behavior and Failure Mechanisms of Freestanding and Composite 3D Graphitic Foams

Open-cell graphitic foams were fabricated by chemical vapor deposition using nickel templates and their compressive responses were measured over a range of relative densities. The mechanical response required an interpretation in terms of a hierarchical micromechanical model, spanning 3 distinct length scales. The power law scaling of elastic modulus and yield strength versus relative density suggests that the cell walls of the graphitic foam deform by bending. The length scale of the unit cell of the foam is set by the length of the struts comprising the cell wall, and is termed level I. The cell walls comprise hollow triangular tubes, and bending of these strut-like tubes involves axial stretching of the tube walls. This length scale is termed level II. In turn, the tube walls form a wavy stack of graphitic layers, and this waviness induces interlayer shear of the graphitic layers when the tube walls are subjected to axial stretch. The thickness of the tube wall defines the third length scale, termed level III. We show that the addition of a thin, flexible ceramic Al2O3 scaffold stiffens and strengthens the foam, yet preserves the power law scaling. The hierarchical model gives fresh insight into the mechanical properties of foams with cell walls made from emergent 2D layered solids.

cond-mat.mtrl-sci↗

arXiv2018

A peeling approach for integrated manufacturing of large mono-layer h-BN crystals

Hexagonal boron nitride (h-BN) is the only known material aside from graphite with a structure composed of simple, stable, non-corrugated atomically thin layers. While historically used as lubricant in powder form, h-BN layers have become particularly attractive as an ultimately thin insulator. Practically all emerging electronic and photonic device concepts rely on h-BN exfoliated from small bulk crystallites, which limits device dimensions and process scalability. Here, we address this integration challenge for mono-layer h-BN via a chemical vapour deposition process that enables crystal sizes exceeding 0.5 mm starting from commercial, reusable platinum foils, and in unison allows a delamination process for easy and clean layer transfer. We demonstrate sequential pick-up for the assembly of graphene/h-BN heterostructures with atomic layer precision, while minimizing interfacial contamination. Our process development builds on a systematic understanding of the underlying mechanisms. The approach can be readily combined with other layered materials and opens a scalable route to h-BN layer integration and reliable 2D material device layer stacks.

cond-mat.mtrl-sci↗

arXiv2018

Raman Spectral Indicators of Catalyst Decoupling for Transfer of CVD Grown 2D Materials

Through a combination of monitoring the Raman spectral characteristics of 2D materials grown on copper catalyst layers, and wafer scale automated detection of the fraction of transferred material, we reproducibly achieve transfers with over 97.5% monolayer hexagonal boron nitride and 99.7% monolayer graphene coverage, for up to 300 mm diameter wafers. We find a strong correlation between the transfer coverage obtained for graphene and the emergence of a lower wavenumber 2D- peak component, with the concurrent disappearance of the higher wavenumber 2D+ peak component during oxidation of the catalyst surface. The 2D peak characteristics can therefore act as an unambiguous predictor of the success of the transfer. The combined monitoring and transfer process presented here is highly scalable and amenable for roll-to-roll processing.

cond-mat.mtrl-sci↗

arXiv2017

Imaging of optically active defects with nanometer resolution

Point defects significantly influence the optical and electrical properties of solid-state materials due to their interactions with charge carriers, which reduce the band-to-band optical transition energy. There has been a demand for developing direct optical imaging methods that would allow in-situ characterization of individual defects with nanometer resolution. Here, we demonstrate the localization and quantitative counting of individual optically active defects in monolayer hexagonal boron nitride using single molecule localization microscopy. By exploiting the blinking behavior of defect emitters to temporally isolate multiple emitters within one diffraction limited region, we could resolve two defect emitters with a point-to-point distance down to ten nanometers. The results and conclusion presented in this work add unprecedented dimensions towards future applications of defects in quantum information processing and biological imaging.

cond-mat.mes-hall↗

arXiv2016

Measuring the Nonlinear Refractive Index of Graphene using the Optical Kerr Effect Method

By means of the ultrafast optical Kerr effect method coupled to optical heterodyne detection (OHD-OKE), we characterize the third order nonlinear response of graphene at telecom wavelength, and compare it to experimental values obtained by the Z-scan method on the same samples. From these measurements, we estimate a negative nonlinear refractive index for monolayer graphene, $n_2 = - 1.1\times 10^{-13} m^2/W$. This is in contradiction to previously reported values, which leads us to compare our experimental measurements obtained by the OHD-OKE and the Z-scan method with theoretical and experimental values found in the literature, and to discuss the discrepancies, taking into account parameters such as doping.

physics.optics↗

arXiv2015

Measuring the Proton Selectivity of Graphene Membranes

By systematically studying the proton selectivity of free-standing graphene membranes in aqueous solutions we demonstrate that protons are transported by passing through defects. We study the current-voltage characteristics of single-layer graphene grown by chemical vapour deposition (CVD) when a concentration gradient of HCl exists across it. Our measurements can unambiguously determine that H+ ions are responsible for the selective part of the ionic current. By comparing the observed reversal potentials with positive and negative controls we demonstrate that the as-grown graphene is only weakly selective for protons. We use atomic layer deposition to block most of the defects in our CVD graphene. Our results show that a reduction in defect size decreases the ionic current but increases proton selectivity.

cond-mat.mes-hall↗

arXiv2014

Graphene-Passivated Nickel as an Oxidation-Resistant Electrode for Spintronics

We report on graphene-passivated ferromagnetic electrodes (GPFE) for spin devices. GPFE are shown to act as spin-polarized oxidation-resistant electrodes. The direct coating of nickel with few layer graphene through a readily scalable chemical vapour deposition (CVD) process allows the preservation of an unoxidized nickel surface upon air exposure. Fabrication and measurement of complete reference tunneling spin valve structures demonstrates that the GPFE is maintained as a spin polarizer and also that the presence of the graphene coating leads to a specific sign reversal of the magneto-resistance. Hence, this work highlights a novel oxidation-resistant spin source which further unlocks low cost wet chemistry processes for spintronics devices.

cond-mat.mtrl-sci↗