20 research outputs found
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
Heat Transport in Layered Semiconductors
No full textHa estat un objectiu de llarga durada dels físics entendre com flueix la calor i com afecta les propietats tèrmiques dels materials. Aquesta recerca ha estat impulsada no només per la recerca del coneixement, sinó també per la gran importància de la calor en la vida diària. El descobriment de nous materials ha introduït desafiaments en la comprensió de propietats fonamentals com el transport de calor, alhora que ha obert camins per a avenços tecnològics.
En aquesta tesi, investiguem les propietats de transport de calor d’una classe de materials laminars coneguts com dicalcogenurs de metalls de transició (TMDs). Estudiem aquests materials utilitzant dues tècniques òptiques diferents: la tècnica convencional de termometria Raman i una nova tècnica de termometria espaciotemporal pump-probe que hem desenvolupat.
Amb aquesta nova tècnica, quantifiquem directament la difusivitat tèrmica en el pla dels TMDs. Les difusivitats tèrmiques obtingudes per a MoSe, fins a un gruix de 3 capes, són consistents amb els resultats obtinguts amb termometria Raman. Curiosament, en mostres monocapa i bicapa, observem una transició en el comportament del transport de calor: des de difusiu en mostres més gruixudes fins a viscós amb molt baixa difusivitat en mostres ultraprimes. Atribuïm aquest transport viscós al flux hidrodinàmic de la calor, un fenomen que mai s’havia observat en TMDs a cap temperatura.Ha sido un objetivo prolongado de los físicos entender cómo fluye el calor y cómo afecta las propiedades térmicas de los materiales. Esta búsqueda ha sido impulsada no solo por la búsqueda del conocimiento, sino también por la tremenda importancia del calor en la vida diaria. El descubrimiento de nuevos materiales ha introducido desafíos en la comprensión de propiedades fundamentales como el transporte de calor, al tiempo que ha abierto avenidas para avances tecnológicos.
En esta tesis, investigamos las propiedades de transporte de calor de una clase de materiales estratificados conocidos como dicalcohenuros de metales de transición (TMDs). Estudiamos estos materiales utilizando dos técnicas ópticas diferentes: la técnica convencional de termometría Raman y una novedosa técnica de termometría de bomba-sonda espaciotemporal que hemos desarrollado.
Con esta nueva técnica, cuantificamos directamente la difusividad térmica en el plano en los TMDs. Las difusividades térmicas obtenidas para MoSe, hasta un grosor de 3 capas, son consistentes con los resultados obtenidos mediante termometría Raman. Interesantemente, en muestras de monocapa y bicapa, observamos una transición en el comportamiento del transporte de calor—de difusivo en muestras más gruesas a viscoso con una difusividad muy baja en muestras ultrafinas. Atribuimos este transporte viscoso al flujo hidrodinámico de calor, un fenómeno que nunca se ha observado en TMDs a ninguna temperatura.It has been a longstanding goal of physicists to understand how heat flows and how it affects the thermal properties of materials. This quest has been driven not only by the pursuit of knowledge but also by the tremendous importance of heat in daily life. The discovery of new materials has introduced challenges in understanding fundamental properties such as heat transport, while also opening avenues for technological advancements.
In this thesis, we investigate the heat transport properties of a class of layered materials known as transition metal dichalcogenides (TMDs). We study these materials using two different optical techniques: the conventional technique of Raman thermometry and a novel spatiotemporal pump-probe thermometry technique that we have developed.
With this new technique, we directly quantify the in-plane thermal diffusivity in TMDs. The thermal diffusivities obtained for MoSe, down to a thickness of 3 layers, are consistent with results obtained from Raman thermometry. Interestingly, in monolayer and bilayer samples, we observe a transition in heat transport behaviour—from diffusive in thicker samples to viscous with very low diffusivity in ultrathin samples. We attribute this viscous transport to the hydrodynamic flow of heat, a phenomenon that has never been observed in TMDs at any temperature.Universitat Autònoma de Barcelona. Programa de Doctorat en Físic
Heat Transport in Layered Semiconductors
No full textHa sido un objetivo prolongado de los físicos entender cómo fluye el calor y cómo afecta las propiedades térmicas de los materiales. Esta búsqueda ha sido impulsada no solo por la búsqueda del conocimiento, sino también por la tremenda importancia del calor en la vida diaria. El descubrimiento de nuevos materiales ha introducido desafíos en la comprensión de propiedades fundamentales como el transporte de calor, al tiempo que ha abierto avenidas para avances tecnológicos. En esta tesis, investigamos las propiedades de transporte de calor de una clase de materiales estratificados conocidos como dicalcohenuros de metales de transición (TMDs). Estudiamos estos materiales utilizando dos técnicas ópticas diferentes: la técnica convencional de termometría Raman y una novedosa técnica de termometría de bomba-sonda espaciotemporal que hemos desarrollado. Con esta nueva técnica, cuantificamos directamente la difusividad térmica en el plano en los TMDs. Las difusividades térmicas obtenidas para MoSe, hasta un grosor de 3 capas, son consistentes con los resultados obtenidos mediante termometría Raman. Interesantemente, en muestras de monocapa y bicapa, observamos una transición en el comportamiento del transporte de calor-de difusivo en muestras más gruesas a viscoso con una difusividad muy baja en muestras ultrafinas. Atribuimos este transporte viscoso al flujo hidrodinámico de calor, un fenómeno que nunca se ha observado en TMDs a ninguna temperatura
Tuning the surface morphology of aluminium doped zinc oxide thin films by arrayed nanorods through chemical growth process
No full textSynthesis and characterization of photoconducting (Cd:Zn)S thin films by hydrothermal assisted chemical bath deposition
No full textA High-throughput Clock-less Architecture for Soft-output Viterbi Detection
No full textViterbi detectors are widely used in data recording channels in the timing loop as well as in the digital back end before error-correction decoding to detect data in the presence of inter-symbol interference (ISI) and noise. Further, soft reliability values assist in the decoding of outer codes. The state-of-the-art implementations of the Viterbi algorithm are synchronous which consider the `worst-case' propagation delays of the combinational blocks for the purpose of timing analysis. This can be avoided by using asynchronous circuits that offer `average case' latencies without a clock distribution network which is one of the most power-consuming units in the existing integrated circuits. In this paper, we present a high-throughput clock-less architecture for a soft-output Viterbi detector. In 180-nm technology node, we obtain a 66.7% reduction in the power consumption for our asynchronous design in comparison to a synchronous version of the detector with throughput requirements of the order of 1.5 Gb/s. Simulation results in 65-nm technology results in 44.2% reduction in power consumption sustaining a throughput of 2.4 Gb/s
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
Transient ultrafast and negative diffusion of charge carriers in suspended MoSe2 from multilayer to monolayer
No full textAbstract Understanding the ultrafast transport properties of charge carriers in transition metal dichalcogenides is essential for advancing technologies based on these materials. Here, we study MoSe2 crystals with thicknesses down to the monolayer, combining ultrafast spatiotemporal microscopy and quantitative microscopic modelling. Crucially, we obtain the intrinsic ultrafast transport dynamics by studying suspended crystals that do not suffer from detrimental substrate effects. In mono- and bilayer crystals, we identify four sequential transport regimes. The first two regimes involve high-energy non-thermalized and quasi-thermalized carriers that propagate rapidly with diffusivities up to 1000 cm2/s. After ~1.5 ps, a remarkable third regime occurs with apparent negative diffusion, finally followed by exciton propagation limited by trapping into defect states. Interestingly, for trilayer and thicker crystals, only the first and last regimes occur. This work underscores the role of traps and dielectric environment in electron transport, offering valuable insights for the development of (flexible) (opto)electronic applications
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
