Experimental signature of a topological quantum dot

Topological insulator nanoparticles (TINPs) host topologically protected Dirac surface states, just like their bulk counterparts. For TINPs of radius <100 nm, quantum confinement on the surface results in the discretization of the Dirac cone. This system of discrete energy levels is referred to as a topological quantum dot (TQD) with energy level spacing on the order of Terahertz (THz), which is tunable with material-Type and particle size. The presence of these discretized energy levels in turn leads to a new electron-mediated phonon-light coupling in the THz range, and the resulting mode can be observed in the absorption cross-section of the TINPs. We present the first experimental evidence of this new quantum phenomenon in Bi2Te3 topological quantum dots, remarkably observed at room temperature. This journal i

Experimental signature of a topological quantum dot

Topological insulators (TIs) present a neoteric class of materials, which support delocalised, conducting surface states despite an insulating bulk. Due to their intriguing electronic properties, their optical properties have received relatively less attention. Even less well studied is their behaviour in the nanoregime, with most studies thus far focusing on bulk samples - in part due to the technical challenges of synthesizing TI nanostructures. We study topological insulator nanoparticles (TINPs), for which quantum effects dominate the behaviour of the surface states and quantum confinement results in a discretized Dirac cone, whose energy levels can be tuned with the nanoparticle size. The presence of these discretized energy levels in turn leads to a new electron-mediated phonon-light coupling in the THz range. We present the experimental realisation of Bi$_2$Te$_3$ TINPs and strong evidence of this new quantum phenomenon, remarkably observed at room temperature. This system can be considered a topological quantum dot, with applications to room temperature THz quantum optics and quantum information technologies

Graphene sensor arrays for rapid and accurate detection of pancreatic cancer exosomes in patients' blood plasma samples

Biosensors based on graphene field effect transistors (GFETs) have the potential to enable the development of point-of-care diagnostic tools for early stage disease detection. However, issues with reproducibility and manufacturing yields of graphene sensors, but also with Debye screening and unwanted detection of nonspecific species, have prevented the wider clinical use of graphene technology. Here, we demonstrate that our wafer-scalable GFETs array platform enables meaningful clinical results. As a case study of high clinical relevance, we demonstrate an accurate and robust portable GFET array biosensor platform for the detection of pancreatic ductal adenocarcinoma (PDAC) in patients’ plasma through specific exosomes (GPC-1 expression) within 45 min. In order to facilitate reproducible detection in blood plasma, we optimized the analytical performance of GFET biosensors via the application of an internal control channel and the development of an optimized test protocol. Based on samples from 18 PDAC patients and 8 healthy controls, the GFET biosensor arrays could accurately discriminate between the two groups while being able to detect early cancer stages including stages 1 and 2. Furthermore, we confirmed the higher expression of GPC-1 and found that the concentration in PDAC plasma was on average more than 1 order of magnitude higher than in healthy samples. We found that these characteristics of GPC-1 cancerous exosomes are responsible for an increase in the number of target exosomes on the surface of graphene, leading to an improved signal response of the GFET biosensors. This GFET biosensor platform holds great promise for the development of an accurate tool for the rapid diagnosis of pancreatic cancer

Atomic Layer Deposition of 2D Metal Dichalcogenides for Electronics, Catalysis, Energy Storage, and Beyond

2D transition metal dichalcogenides (TMDCs) are among the most exciting materials of today. Their layered crystal structures result in unique and useful electronic, optical, catalytic, and quantum properties. To realize the technological potential of TMDCs, methods depositing uniform films of controlled thickness at low temperatures in a highly controllable, scalable, and repeatable manner are needed. Atomic layer deposition (ALD) is a chemical gas-phase thin film deposition method capable of meeting these challenges. In this review, the applications evaluated for ALD TMDCs are systematically examined, including electronics and optoelectonics, electrocatalysis and photocatalysis, energy storage, lubrication, plasmonics, solar cells, and photonics. This review focuses on understanding the interplay between ALD precursors and deposition conditions, the resulting film characteristics such as thickness, crystallinity, and morphology, and ultimately device performance. Through rational choice of precursors and conditions, ALD is observed to exhibit potential to meet the varying requirements of widely different applications. Beyond the current state of ALD TMDCs, the future prospects, opportunities, and challenges in different applications are discussed. The authors hope that the review aids in bringing together experts in the fields of ALD, TMDCs, and various applications to eventually realize industrial applications of ALD TMDCs.Peer reviewe

Room-temperature growth of colloidal Bi₂ Te₃ nanosheets

In this work, we report the colloidal synthesis of Bi2Te3 nanosheets with controlled thickness, morphology and crystallinity at temperatures as low as 20 °C. Grown at room temperature, Bi2Te3 exhibits two-dimensional morphology with thickness of 4 nm and lateral size of 200 nm. Upon increasing the temperature to 170 °C, the nanosheets demonstrate increased thickness of 16 nm and lateral dimensions of 600 nm where polycrystalline nanosheets (20 °C) are replaced by single crystal platelets (170 °C). Rapid synthesis of the material at moderately low temperatures with controllable morphology, crystallinity and consequently electrical and thermal properties can pave the way toward its large-scale production for practical applications

Synthesis of mono- and few-layered n-type WSe2 from solid state inorganic precursors

Tuning the charge transport properties of two-dimensional transition metal dichalcogenides (TMDs) is pivotal to their future device integration in post-silicon technologies. To date, co-doping of TMDs during growth still proves to be challenging, and the synthesis of doped WSe2, an otherwise ambipolar material, has been mainly limited to p-doping. Here, we demonstrate the synthesis of high-quality n-type monolayered WSe2 flakes using a solid-state precursor for Se, zinc selenide. n-Type transport has been reported with prime electron mobilities of up to 10 cm2 V−1 s−1. We also demonstrate the tuneability of doping to p-type transport with hole mobilities of 50 cm2 V−1 s−1 after annealing in air. n-Doping has been attributed to the presence of Zn adatoms on the WSe2 flakes as revealed by X-ray photoelectron spectroscopy (XPS), spatially resolved time of flight secondary ion mass spectroscopy (SIMS) and angular dark-field scanning transmission electron microscopy (AD-STEM) characterization of WSe2 flakes. Monolayer WSe2 flakes exhibit a sharp photoluminescence (PL) peak at room temperature and highly uniform emission across the entire flake area, indicating a high degree of crystallinity of the material. This work provides new insight into the synthesis of TMDs with charge carrier control, to pave the way towards post-silicon electronics

Thickness-Dependent Characterization of Chemically Exfoliated TiS₂ Nanosheets

Monolayer TiS₂ is the lightest member of the transition metal dichalcogenide family with promising applications in energy storage and conversion systems. The use of TiS₂ has been limited by the lack of rapid characterization of layer numbers via Raman spectroscopy and its easy oxidation in wet environment. Here, we demonstrate the layer-number-dependent Raman modes for TiS₂. 1T TiS₂ presents two characteristics of the Raman active modes, A_1g (out-of-plane) and E_g (in-plane). We identified a characteristic peak frequency shift of the E_g mode with the layer number and an unexplored Raman mode at 372 cm^–1 whose intensity changes relative to the A_1g mode with the thickness of the TiS₂ sheets. These two characteristic features of Raman spectra allow the determination of layer numbers between 1 and 5 in exfoliated TiS₂. Further, we develop a method to produce oxidation-resistant inks of micron-sized mono- and few-layered TiS₂ nanosheets at concentrations up to 1 mg/mL. These TiS₂ inks can be deposited to form thin films with controllable thickness and nanosheet density over square centimeter areas. This opens up pathways for a wider utilization of exfoliated TiS₂ toward a range of applications

MoS₂/WS₂ Heterojunction for Photoelectrochemical Water Oxidation

The solar-assisted oxidation of water is an essential half reaction for achieving a complete cycle of water splitting. The search of efficient photoanodes that can absorb light in the visible range is of paramount importance to enable cost-effective solar energy-conversion systems. Here, we demonstrate that atomically thin layers of MoS₂ and WS₂ can oxidize water to O₂ under incident light. Thin films of solution-processed MoS₂ and WS₂ nanosheets display <i>n</i>-type positive photocurrent densities of 0.45 mA cm^–2 and O₂ evolution under simulated solar irradiation. WS₂ is significantly more efficient than MoS₂; however, bulk heterojunctions (B-HJs) of MoS₂ and WS₂ nanosheets results in a 10-fold increase in incident-photon-to-current-efficiency, compared to the individual constituents. This proves that charge carrier lifetime is tailorable in atomically thin crystals by creating heterojunctions of different compositions and architectures. Our results suggest that the MoS₂ and WS₂ nanosheets and their B-HJ blend are interesting photocatalytic systems for water oxidation, which can be coupled with different reduction processes for solar-fuel production