33 research outputs found
High-mobility and high-optical quality atomically thin WS2
No full textThe rise of atomically thin materials has the potential to enable a paradigm shift in modern technologies by introducing multi-functional materials in the semiconductor industry. To date the growth of high quality atomically thin semiconductors (e.g. WS2) is one of the most pressing challenges to unleash the potential of these materials and the growth of mono- or bi-layers with high crystal quality is yet to see its full realization. Here, we show that the novel use of molecular precursors in the controlled synthesis of mono- and bi-layer WS2 leads to superior material quality compared to the widely used direct sulfidization of WO3-based precursors. Record high room temperature charge carrier mobility up to 52 cm2/Vs and ultra-sharp photoluminescence linewidth of just 36 meV over submillimeter areas demonstrate that the quality of this material supersedes also that of naturally occurring materials. By exploiting surface diffusion kinetics of W and S species adsorbed onto a substrate, a deterministic layer thickness control has also been achieved promoting the design of scalable synthesis routes
Ultrafast Tunable Terahertz-to-Visible Light Conversion through Thermal Radiation from Graphene Metamaterials
No full textSeveral technologies, including photodetection, imaging, and data communication, could greatly benefit from the availability of fast and controllable conversion of terahertz (THz) light to visible light. Here, we demonstrate that the exceptional properties and dynamics of electronic heat in graphene allow for a THz-to-visible conversion, which is switchable at a sub-nanosecond time scale. We show a tunable on/off ratio of more than 30 for the emitted visible light, achieved through electrical gating using a gate voltage on the order of 1 V. We also demonstrate that a grating-graphene metamaterial leads to an increase in THz-induced emitted power in the visible range by 2 orders of magnitude. The experimental results are in agreement with a thermodynamic model that describes blackbody radiation from the electron system heated through intraband Drude absorption of THz light. These results provide a promising route toward novel functionalities of optoelectronic technologies in the THz regime.</p
Ultrafast Charge and Exciton Diffusion in Monolayer Films of 9-Armchair Graphene Nanoribbons
No full textDetermining the electronic transport properties of graphene nanoribbons is crucial for assessing their suitability for applications. So far, this has been highly challenging both through experimental and theoretical approaches. This is particularly the case for graphene nanoribbons that are prepared by chemical vapor deposition, which is a scalable and industry-compatible bottom-up growth method that results in closely packed arrays of ribbons with relatively short lengths of a few tens of nanometers. In this study, the experimental technique of spatiotemporal microscopy is applied to study monolayer films of 9-armchair graphene nanoribbons prepared using this growth method, and combined with linear-scaling quantum transport calculations of arrays of thousands of nanoribbons. Both approaches directly resolve electronic spreading in space and time through diffusion and give an initial diffusivity approaching 200 cm2 s−1 during the first picosecond after photoexcitation. This corresponds to a mobility up to 550 cm2 V−1 s−1. The quasi-free carriers then form excitons, which spread with a diffusivity of tens of cm2 s−1. The results indicate that this relatively large charge carrier mobility is the result of electronic transport not being hindered by defects nor inter-ribbon hopping. This confirms their suitability for applications that require efficient electronic transport.</p
A pre-time-zero spatiotemporal microscopy technique for the ultrasensitive determination of the thermal diffusivity of thin films
No full textDiffusion is one of the most ubiquitous transport phenomena in nature. Experimentally, it can be tracked by following point spreading in space and time. Here, we introduce a spatiotemporal pump-probe microscopy technique that exploits the residual spatial temperature profile obtained through the transient reflectivity when probe pulses arrive before pump pulses. This corresponds to an effective pump-probe time delay of 13 ns, determined by the repetition rate of our laser system (76 MHz). This pre-time-zero technique enables probing the diffusion of long-lived excitations created by previous pump pulses with nanometer accuracy and is particularly powerful for following in-plane heat diffusion in thin films. The particular advantage of this technique is that it enables quantifying thermal transport without requiring any material input parameters or strong heating. We demonstrate the direct determination of the thermal diffusivities of films with a thickness of around 15 nm, consisting of the layered materials MoSe2 (0.18 cm2/s), WSe2 (0.20 cm2/s), MoS2 (0.35 cm2/s), and WS2 (0.59 cm2/s). This technique paves the way for observing nanoscale thermal transport phenomena and tracking diffusion of a broad range of species
Graphene-based light sensing: fabrication, characterisation, physical properties and performance
No full textThis is the final version. Available from MDPI via the DOI in this record.Graphene and graphene-based materials exhibit exceptional optical and electrical properties with great promise for novel applications in light detection. However, several challenges prevent the full exploitation of these properties in commercial devices. Such challenges include the limited linear dynamic range (LDR) of graphene-based photodetectors, the lack of efficient generation and extraction of photoexcited charges, the smearing of photoactive junctions due to hot-carriers effects, large-scale fabrication and ultimately the environmental stability of the constituent materials. In order to overcome the aforementioned limits, different approaches to tune the properties of graphene have been explored. A new class of graphene-based devices has emerged where chemical functionalisation, hybridisation with light-sensitising materials and the formation of heterostructures with other 2D materials have led to improved performance, stability or versatility. For example, intercalation of graphene with FeCl3 is highly stable in ambient conditions and can be used to define photo-active junctions characterized by an unprecedented LDR while graphene oxide (GO) is a very scalable and versatile material which supports the photodetection from UV to THz frequencies. Nanoparticles and quantum dots have been used to enhance the absorption of pristine graphene and to enable high gain thanks to the photogating effect. In the same way, hybrid detectors made from stacked sequences of graphene and layered transition-metal dichalcogenides enabled a class of detectors with high gain and responsivity. In this work we will review the performance and advances in functionalised graphene and hybrid photodetectors, with particular focus on the physical mechanisms governing the photoresponse in these materials, their performance and possible future paths of investigation.Funding: M.F.C. and S.R. acknowledge financial support from: Engineering and Physical Sciences Research
Council (EPSRC) of the United Kingdom, projects EP/M002438/1, EP/M001024/1, EPK017160/1, EP/K031538/1,
EP/J000396/1; the Royal Society, grant title "Room temperature quantum technologies" and "Wearable graphene
photovolotaic"; Newton fund, Uk-Brazil exchange grant title "Chronographene" and the Leverhulme Trust,
research grants "Quantum drums" and "Quantum revolution". J.D.M. acknowledges financial support from the
Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom, via the EPSRC Centre for
Doctoral Training in Metamaterials, Grant No. EP/L015331/1
Long lived photogenerated charge carriers in few-layer transition metal dichalcogenides obtained from liquid phase exfoliation
No full textSemiconducting transition metal dichalcogenides are important optoelectronic materials thanks to their intense light-matter interaction and wide selection of fabrication techniques, with potential applications in light harvesting and sensing. Crucially, these applications depend on the lifetimes and recombination dynamics of photogenerated charge carriers, which have primarily been studied in monolayers obtained from labour-intensive mechanical exfoliation or costly chemical vapour deposition. On the other hand, liquid phase exfoliation presents a high throughput and cost-effective method to produce dispersions of mono- and few-layer nanosheets. This approach allows for easy scalability and enables the subsequent processing and formation of macroscopic films directly from the liquid phase. Here, we use transient absorption spectroscopy and spatiotemporally resolved pump-probe microscopy to study the charge carrier dynamics in tiled nanosheet films of MoS and WS deposited from the liquid phase using an adaptation of the Langmuir-Schaefer technique. We find an efficient photogeneration of charge carriers with lifetimes of several nanoseconds, which we ascribe to stabilisation at nanosheet edges. These findings provide scope for photocatalytic and photodetector applications, where long-lived charge carriers are crucial, and suggest design strategies for photovoltaic devices
Fabrication and characterization of large-area suspended MoSe2 crystals down to the monolayer
No full textMany layered materials, such as graphene and transition metal dichalcogenides, can be exfoliated down to atomic or molecular monolayers. These materials exhibit exciting material properties that can be exploited for several promising device concepts. Thinner materials lead to an increased surface-to-volume ratio, with mono- and bi-layers being basically pure surfaces. Thin crystals containing more than two layers also often behave as an all-surface material, depending on the physical property of interest. As a result, flakes of layered materials are typically highly sensitive to their environment, which is undesirable for a broad range of studies and potential devices. Material systems based on suspended flakes overcome this issue, yet often require complex fabrication procedures. Here, we demonstrate the relatively straightforward fabrication of exfoliated MoSe flakes down to the monolayer, suspended over unprecedentedly large holes with a diameter of 15 µm. We describe our fabrication methods in detail, present characterization measurements of the fabricated structures, and, finally, exploit these suspended flakes for accurate optical absorption measurements.The authors would like to thank Emerson Coy (CNBM-AMU) for sharing the HR-TEM images. S V and D S R acknowledge the support of the Spanish Ministry of Economy through FPI-SO 2018 and FPI-SO 2019, respectively. ICN2 was supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). K J T acknowledges funding from the European Union's Horizon 2020 research and innovation program under Grant Agreement No. 804349 (ERC StG CUHL), RyC Fellowship No. RYC-2017-22330, and IAE Project PID2019-111673GB-I00. A E S, E C A, M S and C M S T acknowledge support of the Spanish MICINN Project SIP (PGC2018-101743-B-I00)
High Photoresponsivity İn Graphene-Multilayer Ws2 Heterostructures With An İonic Polymer Gating
No full textEnhanced Thermal Conductivity of Free-Standing Double-Walled Carbon Nanotube Networks
No full textNanomaterials are driving advances in technology due to their oftentimes superior properties over bulk materials. In particular, their thermal properties become increasingly important as efficient heat dissipation is required to realize high-performance electronic devices, reduce energy consumption, and prevent thermal damage. One application where nanomaterials can play a crucial role is extreme ultraviolet (EUV) lithography, where pellicles that protect the photomask from particle contamination have to be transparent to EUV light, mechanically strong, and thermally conductive in order to withstand the heat associated with high-power EUV radiation. Free-standing carbon nanotube (CNT) films have emerged as candidates due to their high EUV transparency and ability to withstand heat. However, the thermal transport properties of these films are not well understood beyond bulk emissivity measurements. Here, we measure the thermal conductivity of free-standing CNT films using all-optical Raman thermometry at temperatures between 300 and 700 K. We find thermal conductivities up to 50 W m-1 K-1 for films composed of double-walled CNTs, which rises to 257 W m-1 K-1 when considering the CNT network alone. These values are remarkably high for randomly oriented CNT networks, roughly seven times that of single-walled CNT films. The enhanced thermal conduction is due to the additional wall, which likely gives rise to additional heat-carrying phonon modes and provides a certain resilience to defects. Our results demonstrate that free-standing double-walled CNT films efficiently dissipate heat, enhancing our understanding of these promising films and how they are suited to applications in EUV lithography.</p
Ultrafast Tunable Terahertz-to-Visible Light Conversion through Thermal Radiation from Graphene Metamaterials
No full textSeveral technologies,
including photodetection, imaging, and data
communication, could greatly benefit from the availability of fast
and controllable conversion of terahertz (THz) light to visible light.
Here, we demonstrate that the exceptional properties and dynamics
of electronic heat in graphene allow for a THz-to-visible conversion,
which is switchable at a sub-nanosecond time scale. We show a tunable
on/off ratio of more than 30 for the emitted visible light, achieved
through electrical gating using a gate voltage on the order of 1 V.
We also demonstrate that a grating-graphene metamaterial leads to
an increase in THz-induced emitted power in the visible range by 2
orders of magnitude. The experimental results are in agreement with
a thermodynamic model that describes blackbody radiation from the
electron system heated through intraband Drude absorption of THz light.
These results provide a promising route toward novel functionalities
of optoelectronic technologies in the THz regime
