Search

Super-resolution fluorescence imaging that surpasses the classical optical resolution limit is widely utilized for resolving the spatial organization of biological structures at molecular length scales. In one example, single-molecule localization microscopy, the lateral positions of single molecules can be determined more precisely than the diffraction limit if the camera collects individual photons separately. Using several schemes that introduce engineered optical aberrations in the imaging optics, super-resolution along the optical axis (perpendicular to the sample plane) has been achieved, and single-molecule localization microscopy has been successfully applied for the study of 3D biological structures. Nonetheless, the achievable axial localization accuracy is typically three to five times worse than the lateral localization accuracy. Only a few exceptional methods based on interferometry exist that reach nanometer 3D super-resolution, but they involve enormous technical complexity and restricted sample preparations that inhibit their widespread application. We developed metal-induced energy transfer imaging for localizing fluorophores along the axial direction with nanometer accuracy, using only a conventional fluorescence lifetime imaging microscope. In metal-induced energy transfer, experimentally measured fluorescence lifetime values increase linearly with axial distance in the range of 0-100 nm, making it possible to calculate their axial position using a theoretical model. If graphene is used instead of the metal (graphene-induced energy transfer), the same range of lifetime values occurs over a shorter axial distance (~25 nm), meaning that it is possible to get very accurate axial information at the scale of a membrane bilayer or a molecular complex in a membrane. Here, we provide a step-by-step protocol for metal- and graphene-induced energy transfer imaging in single molecules, supported lipid bilayer and live-cell membranes. Depending on the sample preparation time, the complete duration of the protocol is 1-3 d.

Hybrid devices consisting of graphene or transition metal dichalcogenides (TMDs) and semiconductor quantum dots (QDs) were widely studied for potential photodetector and photovoltaic applications, while for photodetector applications, high internal quantum efficiency (IQE) is required for photovoltaic applications and enhanced carrier diffusion length is also desirable. Here, we reported the electrical measurements on hybrid field-effect optoelectronic devices consisting of compact QD monolayer at controlled separations from single-layer graphene, and the structure is characterized by high IQE and large enhancement of minority carrier diffusion length. While the IQE ranges from 10.2% to 18.2% depending on QD-graphene separation, ds, the carrier diffusion length, LD, estimated from scanning photocurrent microscopy (SPCM) measurements, could be enhanced by a factor of 5-8 as compared to that of pristine graphene. IQE and LD could be tuned by varying back gate voltage and controlling the extent of charge separation from the proximal QD layer due to photoexcitation. The obtained IQE values were remarkably high, considering that only a single QD layer was used, and the parameters could be further enhanced in such devices significantly by stacking multiple layers of QDs. Our results could have significant implications for utilizing these hybrid devices as photodetectors and active photovoltaic materials with high efficiency.

Combining atomically thin layers of van der Waals (vdW) materials in a chosen vertical sequence is an emerging route to create devices with desired functionalities. While this method aims to exploi...

Twisted van der Waals heterostructures provide a new platform for studying strongly correlated quantum phases. The interlayer coupling in these heterostructures is sensitive to the twist angle (θ) and key to controllably tuning several interesting properties. Here, we demonstrate the systematic evolution of the interlayer coupling strength with twist angle in bilayer MoS2 using a combination of Raman spectroscopy and classical simulations. At zero doping, we observe a monotonic increase in the separation between the A1g and E2g1 mode frequencies as θ decreases from 10°→ 1°, and the separation approaches that of a bilayer at small twist angles. Furthermore, using doping dependent Raman spectroscopy, we reveal the θ dependent softening and broadening of the A1g mode, whereas the E2g1 mode remains unaffected. Using first principles based simulations, we demonstrate large (weak) electron-phonon coupling for the A1g (E2g1) mode, which explains the experimentally observed trends. Our study provides a non-destructive way to characterize the twist angle and the interlayer coupling and establishes the manipulation of phonons in twisted bilayer MoS2 (twistnonics).

Moire superlattices of two-dimensional (2D) materials oriented at low twist angles generate a large-scale interference pattern leading to strong interlayer coupling, which influences the band structure and introduces flatbands. Conventional electronic transport measurements have shown the effects of flatband physics, manifesting as correlated insulating states and emergent superconductivity. In this study, we probe the electronic states in a trinary hybrid of graphene and twisted bilayer (tbl) MoS2. Graphene acts as a sensing layer, which captures the electronic effects of the underlying substrate, and we observe certain anomalies in the electronic characteristics of graphene only in the presence of an underlying 58.5 $$^\circ$$ tbl MoS2, at low temperatures. Interestingly, graphene on tbl MoS2, with twist angle near 0 $$^\circ$$ or on natural bilayer MoS2, does not show any anomalies. Density functional theory calculations show several distinguishable peaks in the density of states at the conduction band edge of twisted MoS2 near 60 $$^\circ$$ . We speculate that the anomaly appears due to fermi level pinning of graphene owing to a large density of states in the flatbands of twisted bilayer MoS2. An analysis of the energetics in the graphene-MoS2 hybrid quantitatively agree with theoretical predictions.

The planar assembly of twisted bilayer graphene (tBLG) hosts multitude of interaction-driven phases when the relative rotation is close to the magic angle (θm = 1.1∘). This includes correlation-induced ground states that reveal spontaneous symmetry breaking at low temperature, as well as possibility of non-Fermi liquid (NFL) excitations. However, experimentally, manifestation of NFL effects in transport properties of twisted bilayer graphene remains ambiguous. Here we report simultaneous measurements of electrical resistivity (ρ) and thermoelectric power (S) in tBLG for several twist angles between θ ~ 1.0 − 1.7∘. We observe an emergent violation of the semiclassical Mott relation in the form of excess S close to half-filling for θ ~ 1.6∘ that vanishes for θ ≳ 2∘. The excess S (≈2 μV/K at low temperatures T ~ 10 K at θ ≈ 1.6∘) persists upto ≈40 K, and is accompanied by metallic T-linear ρ with transport scattering rate (τ−1) of near-Planckian magnitude τ−1 ~ kBT/ℏ. Closer to θm, the excess S was also observed for fractional band filling (ν ≈ 0.5). The combination of non-trivial electrical transport and violation of Mott relation provides compelling evidence of NFL physics intrinsic to tBLG.

We present the recent developments and advances in fluorescence correlation spectroscopy (FCS) and their application to the investigation of biomolecular conformations. In particular, we present and discuss three techniques: multichannel nanosecond FCS, photo-induced electron transfer FCS, and fluorescence lifetime correlation spectroscopy. We briefly describe each method and discuss recent applications to diverse biophysical studies of biomolecular conformation.

The myriad technological applications of infrared radiation sensors make the search for ultra-sensitive detectors extremely crucial. Materials such as bismuth telluride (Bi2Te3), having a small bulk band gap of 0.17 eV, are ideal infrared detectors. However, due to the high recombination rate of photo-generated charge carriers in the bulk, the electrical response under optical illumination is typically very weak in these materials. We have circumnavigated this by sensitizing graphene with Bi2Te3 nano-wires. These hybrid devices show an ultra-high sensitivity of ∼106 A W−1, under incident electromagnetic radiation from 940 nm to 1720 nm. The theoretical limit of the noise equivalent power and specific detectivity in these devices are ∼10−18 W Hz−1/2 and ∼1011 Jones respectively, which are comparable to those of some of the best known detectors.

Engineering the properties of layered metal dichalcogenides (LMDs) requires stringent control of their morphology. Herein, using a scalable one-step solvothermal technique, we report the synthesis of SnSe2 under two different conditions, leading to the formation of nanoflakes and nanoflowers. The use of oleic acid in the reaction leads to the formation of nanoflowers, and the presence of ethanol in the reaction medium leads to the formation of nanoflakes. Ab initio density functional theory calculations rationalise this observation, revealing a stronger adsorption of ethanol on the {0001} facet compared to the acid. Furthermore, these SnSe2 nanoflakes, when integrated with graphene, also respond to incident electromagnetic radiation, from the visible to near infrared regime, with a specific detectivity of ∼5 × 1010 Jones, which is comparable to that of the best available photodetectors, making them suitable for use in various technological applications.

The intriguing electronic properties of graphene and the strong light-matter interaction in layered transition-metal dichalcogenide (TMDC) make them a natural partner for hybrid devices, not only for optoelectronic device applications, but also to understand the conversion of light to electricity in this atomic scale prototype of a donor-acceptor complex. Here, we describe graphene-on-${\mathrm{WS}}_{2}$ binary heterostructure FET device, displaying gate-tunable persistent photoconductivity. Our time-dependent photovoltage relaxation experiments suggest that the charge-transfer time scale in this heterostructure is dependent on the input optical power, contrary to what is observed for the bare TMDC and is orders of magnitude slower than that observed for various other vdW hybrids. The optoelectronic responsivity of this device at low optical power is found to be as high as ${10}^{10}\phantom{\rule{0.1em}{0ex}}\mathrm{V/W}$, and thus shows the potential to be one of the most sensitive visible range photodetectors, while the gate tunability of the persistent photoconductivity can be utilized in the memory device applications. We identify that the photoresponse is the outcome of a photogating mechanism, due to the exciton dissociation under optical excitation, followed by the trapping of holes in ${\mathrm{WS}}_{2}$ and subsequent electron transfer to graphene.

Graphene grain boundaries (GBs) have attracted interest for their ability to host nearly dispersionless electronic bands and magnetic instabilities. Here, we employ quantum transport and universal conductance fluctuation measurements to experimentally demonstrate a spontaneous breaking of time-reversal symmetry across individual GBs of chemical vapor deposited graphene. While quantum transport across the GBs indicate spin-scattering-induced dephasing and hence formation of local magnetic moments, below T4 K we observe complete lifting of time-reversal symmetry at high carrier densities (n5A�1012 cm-2) and low temperature (T2 K). An unprecedented thirtyfold reduction in the universal conductance fluctuation magnitude with increasing doping density further supports the possibility of an emergent frozen magnetic state at the GBs. Our experimental results suggest that realistic GBs of graphene can be a promising resource for new electronic phases and spin-based applications. © 2021 American Physical Society.

Graphene, a two-dimensional nanomaterial, has gained immense interest in biosensing applications due to its large surface-to-volume ratio, and excellent electrical properties. Herein, a compact and user-friendly graphene field effect transistor (GraFET) based ultrasensitive biosensor has been developed for detecting Japanese Encephalitis Virus (JEV) and Avian Influenza Virus (AIV). The novel sensing platform comprised of carboxy functionalized graphene on Si/SiO2 substrate for covalent immobilization of monoclonal antibodies of JEV and AIV. The bioconjugation and fabrication process of GraFET was characterized by various biophysical techniques such as Ultraviolet–Visible (UV–Vis), Raman, Fourier-Transform Infrared (FT-IR) spectroscopy, optical microscopy, Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). The change in the resistance due to antigen–antibody interaction was monitored in real time to evaluate the electrical response of the sensors. The sensors were tested in the range of 1 fM to 1 μM for both JEV and AIV antigens, and showed a limit of detection (LOD) upto 1 fM and 10 fM for JEV and AIV respectively under optimised conditions. Along with ease of fabrication, the GraFET devices were highly sensitive, specific, reproducible, and capable of detecting ultralow levels of JEV and AIV antigen. Moreover, these devices can be easily integrated into miniaturized FET-based real-time sensors for the rapid, cost-effective, and early Point of Care (PoC) diagnosis of JEV and AIV.

The fundamental origin of low-frequency noise in graphene field effect transistors (GFETs) has been widely explored but a generic engineering strategy towards low noise GFETs is lacking. Here, we systematically study and eliminate dominant sources of electrical noise to achieve ultralow noise GFETs. We find that in edge contacted, high-quality hexagonal boron nitride (hBN) encapsulated GFETs, the inclusion of a graphite bottom gate and long (≳1.2 μm) channel-contact distance significantly reduces noise as compared to global Si/SiO2 gated devices. From the scaling of the remaining noise with channel area and its temperature dependence, we attribute this to the traps in hBN. To further screen the charge traps in hBN, we place few layers of MoS2 between graphene and hBN, and demonstrate that the noise is as low as ∼5.2 × 10−9 μm2 Hz−1 (corresponding to minimum Hooge parameter ∼5.2 × 10−6) in GFETs at room temperature, which is an order of magnitude lower than the earlier reported values.

We discuss the effect of the in-plane electric field on the Raman spectroscopy for few-layered MoS2. The characteristic Raman modes of MoS2 show gradual red shift, while the intensity increases by 45–50% as the electric field is increased, showing a large electro-optical effect. Structural analysis suggests that our few-layered MoS2 belongs to P6/m2 space group with broken inversion symmetry. We attribute this gradual red shift to this broken symmetry-driven piezoelectricity in MoS2, which generates tensile strain along the perpendicular direction when the electric field is applied. The enhancement of the effect upon reversing the electric field direction adds credence to our interpretation. Our first principal density-functional theory calculation further substantiates the claim. This optical probing of the electromechanical coupling may lead to applications as a nonextensive technique for electric field/strain sensors in the nanoelectronics devices.

Polyethylene terephthalate-based nanocomposites with hexagonal boron nitride nanosheets (BNNs) were prepared by a solution casting method with varying concentrations of BNNs from 0.5 wt% to 4 wt%. Melting and crystallization behaviour of the composites were investigated by differential scanning calorimetry, which suggests that with increasing presence of nanosheets, the crystallinity increases and hence the polyethylene terephthalate chain mobility gets restricted, which leads to suppression of crystal growth. The nanoindentation measurements on the composite films exhibit improved mechanical properties. Enhancement of 33.3% of elastic modulus and 32.4% of hardness was observed with 2 wt% infusion of boron nitride nanosheets in polyethylene terephthalate.

There have been several reports of plasmonically enhanced graphene photodetectors in the visible and the near infrared regime but rarely in the ultraviolet. In a previous work, we have reported that a graphene-silver hybrid structure shows a high photoresponsivity of 13 A/W at 270 nm. Here, we consider the likely mechanisms that underlie this strong photoresponse. We investigate the role of the plasmonic layer and examine the response using silver and gold nanoparticles of similar dimensions and spatial arrangement. The effect on local doping, strain, and absorption properties of the hybrid is also probed by photocurrent measurements and Raman and UV-visible spectroscopy. We find that the local doping from the silver nanoparticles is stronger than that from gold and correlates with a measured photosensitivity that is larger in devices with a higher contact area between the plasmonic nanomaterials and the graphene layer.

Resolving low-energy features in the density of states (DOS) holds the key to understanding wide variety of rich novel phenomena in graphene based 2D heterostructures. Lifshitz transition in bilayer graphene (BLG) arising from trigonal warping has been established theoretically and experimentally. Nevertheless, the experimental realization of its effects on the transport properties has been challenging because of its relatively low energy scale ($\sim 1$ meV). In this work, we demonstrate that the thermoelectric power (TEP) can be used as an effective probe to investigate fine changes in the DOS of BLG. We observe additional entropy features in the vicinity of the charge neutrality point (CNP) in gapped BLG. This apparent violation of Mott formula can be explained quantitatively by considering the effects of trigonal warping, thereby serving as a possible evidence of a Lifshitz transition.

Doping of organic semiconductors enhances the performance of optoelectronic devices. Although p-type doping is well studied and successfully deployed in optoelectronic devices, air stable ntype doping was still elusive. We succeeded with n-type doping of organic semiconductors using molecular dopant N-DMBI under ambient conditions. Strikingly, n-type doping accounts for a gigantic increase of the photoconductivity of doped thin films. Electrical and optical properties of the n-doped molecular semiconductor were investigated by temperature dependent conductivity, electron paramagnetic resonance (EPR), and flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements. A significant reduction and saturation in activation energy with increasing doping level clearly suggests the formation of an impurity band and enhancement in carrier density. Computational studies reveal the formation of a charge transfer complex mediated by hydrogen abstraction as the rate-determining step for the doping mechanism. The colossal enhancement of photoconductivity induced by n-doping is a significant step toward optoelectronic devices made of molecular semiconductors.

The existence of an exquisite phenomenon such as a metal-insulator transition (MIT) in two-dimensional (2D) systems, where completely different electronic functionalities in the same system can emerge simply by regulating parameters such as charge carrier density in them, is noteworthy. Such tunability in material properties can lead to several applications where precise tuning of function specific properties are desirable. Here, we report on our observation on the occurrence of MIT in the 2D material system of copper indium selenide (CuIn

The introduction of 'twist' or relative rotation between two atomically thin van der Waals (vdW) membranes gives rise to periodic Moire potential, leading to a substantial altercation of the band structure of the planar assembly. While most of the recent experiments primarily focus on the electronic-band hybridization by probing in-plane transport properties, here we report out-of-plane thermoelectric measurements across the van der Waals gap in twisted bilayer graphene (tBLG), which exhibits an interplay of twist-dependent inter-layer electronic and phononic hybridization. We show that at a large twist angle, the thermopower is entirely driven by a novel phonon drag effect at the sub-nanometer scale, while the electronic component of the thermopower is recovered only when the misorientation between the layers is reduced to less than two degrees. Our experiment shows that cross-plane thermoelectricity at a low angle is exceptionally sensitive to the nature of band dispersion and may provide fundamental insights into the coherence of electronic states in twisted bilayer graphene., 5 figures

The mechanism of electrical noise in transition metal dichalcogenides (TMDCs) has mostly been attributed to charge carrier fluctuations between the oxide traps and the conducting channel, in accordance with the McWhorter model. However, the original McWhorter model was formulated for diffusive transport with conducting carriers having extended electronic wave functions. Our work serves to generalize the McWhorter mechanism to include strongly localized systems such as the TMDC family and provides an explanation for the unusual exponential behavior of noise magnitude with temperature.

Many complex luminescent emitters such as fluorescent proteins exhibit multiple emitting states that result in rapid fluctuations of their excited-state lifetime. Here, we apply fluorescence lifetime correlation spectroscopy (FLCS) to resolve the photophysical state dynamics of the prototypical fluorescence protein enhanced green fluorescent protein (EGFP). We quantify the microsecond transition rates between its two fluorescent states, which have otherwise highly overlapping emission spectra. We relate these transitions to a room-temperature angstrom-scale rotational isomerism of an amino acid next to its fluorescent center. With this study, we demonstrate the power of FLCS for studying the rapid transition dynamics of a broad range of light-emitting systems with complex multistate photophysics, which cannot be easily done by other methods.

Van der Waals (vdW) heterostructure is a type of metamaterial where multiple layers of 2D materials are vertically aligned at controlled misorientation. The relative rotation in between the adjacent layers, or the twist angle between them plays a crucial role in changing the electronic band structure of the superlattice. The assembly of multi-layers of precisely twisted two dimensional layered materials requires knowledge of the atomic structure at the edge of the flake. It may be artificially created by the ‘tear and stack’ process. Otherwise, the crystallographic orientation needs to be determined through invasive processes such as transmission electron microscopy or scanning tunneling microscopy, and via second-harmonic generation (SHG). Here, we demonstrate a simple and elegant transfer protocol using only an optical microscope as a edge identifier tool through which, controlled transfer of twisted homobilayer and heterobilayer transition metal dichalcogenides is performed with close to 100% yield. The fabricated twisted vdW heterostructures have been characterized by SHG, Raman spectroscopy and photoluminiscence spectroscopy, confirming the desired twist angle within ∼0.5° accuracy. The presented method is reliable, quick and prevents the use of invasive tools which is desirable for reproducible device functionalities.

Despite its importance in the large-scale synthesis of transition metal dichalcogenides (TMDC) molecular layers, the generic quantum effects on electrical transport across individual grain boundaries (GBs) in TMDC monolayers remain unclear. Here we demonstrate that strong carrier localization due to the increased density of defects determines both temperature dependence of electrical transport and low-frequency noise at the GBs of chemical vapor deposition (CVD)-grown MoS2 layers. Using field effect devices designed to explore transport across individual GBs, we show that the localization length of electrons in the GB region is ∼30–70% lower than that within the grain, even though the room temperature conductance across the GB, oriented perpendicular to the overall flow of current, may be lower or higher than the intragrain region. Remarkably, we find that the stronger localization is accompanied by nearly 5 orders of magnitude enhancement in the low-frequency noise at the GB region, which increases expon...

Dielectric spectroscopy (DS) is a non‐invasive, label‐free, and promising technique for measuring dielectric properties of biological cells. Recent developments in microfabrication techniques made it possible to perform DS measurements with minute volume of cell suspensions. However, when the cell size approaches the size of the measurement chamber, especially, for single cell measurements, the analytical models [Maxwell–Wagner and Bruggeman–Hanai (BH) mixture models] to extract cell parameters lose their accuracy. Moreover, variations in the cell position relative to the measurement electrodes decrease the accuracy of the analytical solutions. Impedance spectrum of a typical eukaryotic mammalian cell is generated for different geometrical configurations using finite element. The generated data are fitted to the analytical models and extracted cell parameters are compared with the original values. The results show that BH model works more effectively when chamber to cell radius ratio is

Despite the many fascinating discoveries of fundamental significance and device applications involving graphene, one area that has been lacking is graphene-based displays and emissive devices. Since graphene by itself has weak and wavelength-independent absorption and no emission in the visible range, such devices must rely on synergistic combination with other highly sensitive optical materials such as quantum dots. However, the well-known strong nonradiative energy transfer between emitters and quantum dots and graphene makes it impossible to create such devices due to strong emission quenching. Here we report the first demonstration of enhanced photoluminescence of quantum dots in close proximity to graphene field effect transistor devices, which are electrically and spectrally tunable. The enhanced emission originates from super-radiance between closely packed quantum dots placed close to single-layer graphene, which overcomes the strong nonradiative quenching observed earlier. Finite difference time ...

The weak interlayer coupling in van der Waals solids allows isolation of individual atomic or molecular layers with remarkable electrical, optical, and structural properties. The applicability of these two dimensional materials in future electronic architectures requires a thorough understanding of the electrical transport mechanisms as well as of the factors limiting the device performance. The study of slow time-varying fluctuations in resistance, often called the 1 / f noise, offers deep insight into the electronic transport and scattering mechanisms, and also acts as a performance benchmark. Here we review the current status on the magnitude and microscopic understanding of low-frequency 1 / f noise in two-dimensional electronic materials, with specific focus on graphene, bismuth chalcogenides, and transition metal dichalcogenides. The noise characteristics differ significantly among these systems, and are directly influenced by the band structure, energy dispersion, and screening properties. The noise in field effect devices from van der Waals materials closely compares with or is even lower than that of conduction channels in conventional electronics. The excellent noise performance, when combined with high carrier mobility, energy efficiency and structural flexibility, renders an excellent platform for future electronic, optoelectronic, and sensor applications.

Several factors can affect performance of an ultrasound system such as quality of excitation signal and ultrasound transducer behaviour. Nonlinearity of piezoelectric ultrasound transducers is a key determinant in designing a proper driving power supply. Although, the nonlinearity of piezoelectric transducer impedance has been discussed in different literatures, the trend of the nonlinearity at different frequencies with respect to excitation voltage variations has not been clearly investigated in practice. In this paper, to demonstrate how the nonlinearity behaves, a sandwich piezoceramic transducer was excited at different frequencies. Different excitation signals were generated using a linear power amplifier and a multilevel converter within a range of 30–200 V. Empirical relation was developed to express the resistance of the piezoelectric transducer as a nonlinear function of both excitation voltage and resonance frequency. The impedance measurements revealed that at higher voltage ranges, the piezoelectric transducer can be easily saturated. Also, it was shown that for the developed ultrasound system composed of two transducers (one transmitter and one receiver), the output voltage measured across receiver is a function of a voltage across the resistor in the RLC branches and is related to the resonance frequencies of the ultrasound transducer.

The optoelectronic performance of hybrid devices from graphene and optically sensitive semiconductors exceeds conventional photodetectors due to a large in-built optical gain. Tellurium nanowire (TeNW), being a narrow direct band gap semiconductor (∼0.65 eV), is as an excellent potential candidate for near infra-red (NIR) detection. Here we demonstrate a new graphene-TeNW binary hybrid that exhibits a maximum photoresponsivity of ∼106 A W-1 at 175 K in the NIR regime (920 nm-1720 nm), which exceeds the photoresponsivity of the most common NIR photodetectors. The resulting noise-equivalent power (NEP) is as low as 2 × 10-18 W Hz-1/2, and the specific detectivity (D*) exceeds 5 × 1013 cm Hz1/2 W-1 (Jones). The temperature range of optimal operation, which extends up to ≈220 K and ≈260 K for 1720 nm and 920 nm excitation, respectively, is primarily limited by the electrical conductivity of the TeNW layer, and can further be improved by lowering of the defect density as well as inter-wire electronic coupling.

Graphene constitutes one of the key elements in many functional van der Waals heterostructures. However, it has negligible optical visibility due to its monolayer nature. Here we study the visibility of graphene in various van der Waals heterostructures and include the effects of the source spectrum, oblique incidence and the spectral sensitivity of the detector to obtain a realistic model. A visibility experiment is performed at different wavelengths, resulting in a very good agreement with our calculations. This allows us to reliably predict the conditions for better visibility of graphene in van der Waals heterostructures. The framework and the codes provided in this work can be extended to study the visibility of any 2D material within an arbitrary van der Waals heterostructure.

In recent years single-molecule localization super-resolution microscopy (SMLM) has become an indispensable tool for many fields of research. Here, for any image of a single molecule one determines its center position with much higher accuracy than the size of that image itself. A challenge of SMLM is to achieve super-resolution also along the third dimension. Recently, Metal-Induced Energy Transfer or MIET [1,2] was introduced. It exploits the energy transfer from an excited fluorophore to plasmons in a thin metal film. Similar to Forster Resonance Energy Transfer (FRET), this coupling shows a strong distance dependence, but over a range up to 150 nm and enables axial localization of fluorophores with 5-6 nm resolution at a photon budget of 1000 photons. [3,4] Here, we show that using a graphene layer the localization accuracy of MIET reaches Angstrom accuracy. At such accuracy, minute details such as nanometer-scale roughness of the sample surfaces becomes important. For proof of principle, we determined absolute distances of single molecules from a surface for samples with an a priori well-known sample geometry. We spin-coated fluorescent dye molecules (Atto655) on top of three different substrates with spacer thickness values of 10, 15, and 20 nm, defining the distance of the molecules from the graphene layer. Next, we determined the thickness of supported lipid bilayers (SLBs) by localizing fluorescent dyes attached to lipid head groups in the bottom and top leaflet of the SLB. We have demonstrated that by using graphene as the energy acceptor in MIET, the axial localization accuracy and resolution reaches sub-nanometer levels at photon budgets which are typical in conventional SMLM experiments. An interesting feature of graphene-MIET is that it provides an axial localization accuracy which now surpasses significantly that of lateral localization provided by most SMLM approaches. [1] A. I. Chizhik et al. Nat. Photonics 8, 124 (2014). [2] S. Isbaner et al. Nano Lett. 18, 2616 (2018). [3] N. Karedla et al. ChemPhysChem 15, 705 7 (2014). [4] N. Karedla et al. J. Chem. Phys. 148, 204201 (2018).

(CoFe2O4)x(BaFe12O19)1-x (x= 0.0, 0.4, 0.6, 0.8, 1.0) nano-composites was synthesized via single reaction mixture, utilizing sol-gel auto combustion method, with an intention to have superior structural homogeneity, and eventually to have single magnetic phase with enhanced exchange coupling between CoFe2O4 (soft) − BaFe12O19 (hard) phase. As-synthesized composite was thermally treated at 800 °C/4hrs, needed for the formation of hexagonal phase. X-ray diffraction (XRD) displays presence of both soft and hard phases only. Grain size for BaFe12O19 and CoFe2O4 respectively range between 50.2 − 54.5 nm and 45.5 − 56.5 nm. Obtained value of saturation magnetization (Ms), coercivity (Hc) for the studied samples is respectively: 39.8 Am2/kg − 75.5 Am2/kg and 501.2 Oe − 2587.9 Oe. Observed Ms, Hc values clearly show that single reaction mixture leads to better exchange coupling between soft-hard phases. Ms enhancement was also observed with increasing substitution amount (x). Observed results point out (CoFe2O4)x(BaFe12O19)1-x composite system as a good candidate for their potential applications as hard magnets.

Magnetite (Fe3O4) nano particles have been synthesized via sol-gel auto-combustion method using citric acid as a fuel. Post preparation thermal treatment was done at: 220 °C, 270 °C and, 320 °C for 1 hour, to insure the formation of single phase Fe3O4 nano particles. X-Ray diffraction (XRD) results confirm the formation of Magnetite (Fe3O4), and also the presence of Hematite (α-Fe2O3) as secondary phase. Results reveal that with increasing annealing temperature lattice parameter (a), specific surface area (S), hopping lengths (LA, LB) for tetrahedral (A) and octahedral site (B) increases, whereas x-ray density (ρXRD) and, Scherer’s grain diameter (Ds) varies respectively range between 5286.2−5376.1 kg/m3, 37.2−43.5 nm. Study of Fe3O4 nano particles is of value, due to its prospective applications in medical science - treating metastatic cancer by injecting into the cells.

The Zinc oxide thin films were grown on the amorphous glass substrate using Successive Ionic Layer Adsorption and Reaction (SILAR) technique at room temperature. The various preparative parameters were optimized to obtain good quality films. The as-grown films were annealed at 350° C for 2 h in air. The comparative study of the annealed and the as-grown thin films of ZnO were done for structural, optical and electrical studies. Polycrystalline nature was depicted from the x-ray diffraction (XRD) patterns. The peak intensity of as-grown ZnO thin films enhances after annealing. The average grain size calculated from XRD patterns was 17.66 nm for as-grown, while it increased to 24.66 nm after annealing. The band gap of as-grown ZnO thin films was relatively higher and was found to be 3.80 eV. After air annealing, the band gap decrease to 3.58 eV. The current-voltage characteristics show nearly ohmic behavior with decrease in the film resistance, after annealing in air. N-type conductivity was confirmed from the TEP measurements.The Zinc oxide thin films were grown on the amorphous glass substrate using Successive Ionic Layer Adsorption and Reaction (SILAR) technique at room temperature. The various preparative parameters were optimized to obtain good quality films. The as-grown films were annealed at 350° C for 2 h in air. The comparative study of the annealed and the as-grown thin films of ZnO were done for structural, optical and electrical studies. Polycrystalline nature was depicted from the x-ray diffraction (XRD) patterns. The peak intensity of as-grown ZnO thin films enhances after annealing. The average grain size calculated from XRD patterns was 17.66 nm for as-grown, while it increased to 24.66 nm after annealing. The band gap of as-grown ZnO thin films was relatively higher and was found to be 3.80 eV. After air annealing, the band gap decrease to 3.58 eV. The current-voltage characteristics show nearly ohmic behavior with decrease in the film resistance, after annealing in air. N-type conductivity was confirmed from ...

Amongst various metal oxides, Zinc Oxide (ZnO) is widely used semiconducting material amongst various II-VI group materials due to its various properties like wide energy band gap (3.3 eV), strong binding energy of 60 meV, higher transparency, low electron affinity sensitivity and selectivity for device grade applications. For this study purpose, for this study purpose (ZnO) thin films have been deposited by Successive Ionic Layer Adsorption and Reaction (SILAR) technique on amorphous glass substrate at room temperature. These films were then annealed for about 2 hours in air atmosphere. The structural studies reveal the increase in grain size upto 400°C. The change in crystallite size were observed in the Atomic Force Microscopy (AFM) images. The optical studies were also in well agreement with the structural analysis.

Amongst various semiconductors, Bi2S3 is one of the most widely used compounds, used for various types of applications like thermoelectric and optoelectronic applications as well as biological and chemical sensors due to its various types of properties. Bi2S3 is a challenging material because of its midway band gap (Eg=1.74eV) and absorption coefficient of the order of 104 cm-1. There are a lot of techniques have developed for synthesizing the Bi2S3 thin films. Amongst them, Zhehao Zhu et al have revealed that as annealing temperature increases in sulfurvapour atmosphere, the light absorption and charge separation efficiencies enhances by achieving carrier diffusion length that is comparable to the light absorption depth, leading to high solar energy conversion efficiencies in Bi2S3. R.S. Mane et al also have reported that, the Photoelectrochemical (PEC) performance of chemically grown Bi2S3 is improved with increase in film thickness and grain size.

Soft−hard (CoFe2O4)x(SrFe12O19)1-x (0.0≤x ≤1.0) nano-composite(s) were prepared by novel one pot reaction sol-gel auto-combustion route and annealed at 800 °C for 4 hours. X-ray diffraction (XRD), and vibration sample magnetometer (VSM) were utilized to monitor the effect of substitution on structural, and magnetic properties. XRD confirms the existence of soft (CoFe2O4), and hard (SrFe12O19) phase with average grain size (D) ranging between 57.7 nm to 59.4 nm for hexagonal and 34.6 to 64.5 nm for cubic phase. The magnetization of nano-composites demonstrates hysteresis loop that is characteristic of the exchange coupled system showing structurally two phases, but magnetically single phase. As a result of exchange coupling, between Co, and Sr ferrite enhanced saturation magnetization (70.65 Am2/kg), and remanence (26.29 Am2/kg) were observed as compared to pure SrFe12O19. Observed enhancement in energy product (BH)max (0.83 − 28.96 kJ/m3) of nano composite also favors good exchange coupling between hard and soft phase.

Sintered pellets of zinc oxide (ZnO), both undoped and Al-doped are prepared through a chemical process. Dopant concentration of Aluminium in ZnO [Al/Zn in weight percentage (wt%)] is varied from 0 to 3 wt%. After synthesis structural characterisation of the samples are performed with XRD and SEM-EDAX which confirm that all the samples are of ZnO having polycrystalline nature with particle size from 108.6 to 116 nm. Frequency dependent properties like a.c. conductivity, capacitance, impedance and phase angle are measured in the frequency range 10 Hz to 100 kHz as a function of temperature (in the range 25–150 °C). Nature of a.c. conductivity in these samples indicates hopping type of conduction arising from localised defect states. The frequency and temperature dependent properties under study are found to be as per correlated barrier hoping model. Dielectric and impedance properties studied in the samples indicate distributed relaxation, showing decrease of relaxation time with temperature.

The transfer of charge carriers across the optically excited hetero-interface of graphene and semiconducting transition metal dichalcogenides (TMDCs) is the key to convert light to electricity, although the intermediate steps from the creation of excitons in TMDC to the collection of free carriers in the graphene layer are not fully understood. Here, we investigate photo-induced charge transport across graphene–MoS2 and graphene–WSe2 hetero-interfaces using time-dependent photoresistance relaxation with varying temperature, wavelength, and gate voltage. In both types of heterostructures, we observe an unprecedented resonance in the inter-layer charge transfer rate as the Fermi energy ( E F) of the graphene layer is tuned externally with a global back gate. We attribute this to a resonant quantum tunneling from the excitonic state of the TMDC to E F of the graphene layer and outline a new method to estimate the excitonic binding energies ( E b) in the TMDCs, which are found to be 400 meV and 460 meV in MoS2 and WSe2 layers, respectively. The gate tunability of the inter-layer charge transfer timescales may allow precise engineering and readout of the optically excited electronic states at graphene–TMDC interfaces.

We report the observation of linear magneto-resistance (LMR) with a very low cross-over magnetic field (Bc ) at room temperature in Bismuth(110) thin films fabricated using thermal evaporation. The magneto-resistance at different temperatures (T) indicates that Bc has a strong T - dependence, and displays a remarkably small magnitude of 180 mT at T = 300 K. The parameter Φ, defined as the MR% (B = 1 T)/ Bc is 248, is considerably higher at room temperature compared to values previously reported in literature. The observed LMR is likely due to the recombination of carriers in compensated systems near charge neutrality. Our measurements demonstrate that the LMR can be observed at a very low field and high T , which can be utilized in a variety of applications.

Strong optoelectronic response in the binary van der Waals heterostructures of graphene and transition metal dichalcogenides (TMDCs) is an emerging route towards high-sensitivity light sensing. While the high sensitivity is an effect of photogating of graphene due to inter-layer transfer of photo-excited carriers, the impact of intrinisic defects, such as traps and mid-gap states in the chalcogen layer remain largely unexplored. Here we employ graphene/hBN (hexagonal boron nitride)/MoS2 (molybdenum disulphide) trilayer heterostructures to explore the photogating mechanism, where the hBN layer acts as interfacial barrier to tune the charge transfer timescale. We find two new features in the photoresponse: First, an unexpected positive component in photoconductance upon illumination at short times that preceeds the conventional negative photoconductance due to charge transfer, and second, a strong negative photoresponse at infrared wavelengths (up to 1720 nm) well-below the band gap of single layer MoS2. Detailed time and gate voltage-dependence of the photoconductance indicates optically-driven charging of trap states as possible origin of these observations. The responsivity of the trilayer structure in the infrared regime was found to be extremely large (> 108 A/W at 1550 nm using 20 mV source drain bias at 180 K temperature and ≈ −30 V back gate voltage). Our experiment demonstrates that interface engineering in the optically sensitive van der Waals heterostructures may cast crucial insight onto both inter- and intra-layer charge reorganization processes in graphene/TMDC heterostructures.

The diverse properties of two-dimensional materials have been utilized in a variety of architecture to fabricate high quality electronic circuit elements. Here we demonstrate a generic method to control hysteresis and stable memory effect in Van der Waals hybrids with a floating gate as the base layer. The floating gate can be charged with a global back gate-voltage, which it can retain in a stable manner. Such devices can provide a very high, leakage-free effective gate-voltage on the field-effect transistors due to effective capacitance amplification, which also leads to reduced input power requirements on electronic devices. The capacitance amplification factor of ∼10 can be further enhanced by increasing the area of the floating gate. We have exploited this method to achieve highly durable memory action multiple genre of ultra-thin 2D channels, including graphene, MoS2, and topological insulators at room temperature.

The weak light–matter interaction in graphene can be enhanced with a number of strategies, among which sensitization with plasmonic nanostructures is particularly attractive. This has resulted in the development of graphene–plasmonic hybrid systems with strongly enhanced photodetection efficiencies in the visible and the IR, but none in the UV. Here, we describe a silver nanoparticle–graphene stacked optoelectronic device that shows strong enhancement of its photoresponse across the entire UV spectrum. The device fabrication strategy is scalable and modular. Self-assembly techniques are combined with physical shadow growth techniques to fabricate a regular large-area array of 50 nm silver nanoparticles onto which CVD graphene is transferred. The presence of the silver nanoparticles resulted in a plasmonically enhanced photoresponse as high as 3.2 A W−1 in the wavelength range from 330 nm to 450 nm. At lower wavelengths, close to the Van Hove singularity of the density of states in graphene, we measured an even higher responsivity of 14.5 A W−1 at 280 nm, which corresponds to a more than 10 000-fold enhancement over the photoresponse of native graphene.

The out-of-plane electric polarization at the surface of SrTiO3 (STO), an archetypal perovskite oxide, may stabilize new electronic states and/or host novel device functionality. This is particularly significant in proximity to atomically thin membranes, such as graphene, although a quantitative understanding of the polarization across graphene–STO interface remains experimentally elusive. Here, we report direct observation and measurement of a large intrinsic out-of-plane polarization at the interface of single-layer graphene and TiO2-terminated STO (100) crystal. Using a unique temperature dependence of anti-hysteretic gate-transfer characteristics in dual-gated graphene-on-STO field-effect transistors, we estimate the polarization to be as large as ≈12 μC cm−2, which is also supported by the density functional theory calculations and low-frequency noise measurements. The anti-hysteretic transfer characteristics is quantitatively shown to arise from an interplay of band bending at the STO surface and electrostatic potential due to interface polarization, which may be a generic feature in hybrid electronic devices from two-dimensional materials and perovskite oxides. The dipole field at the surface of SrTiO3 strongly impacts the transfer characteristics of graphene/SrTiO3 field-effect transistors. A team led by Arindam Ghosh at the Indian Institute of Science (Bangalore) fabricated dual-gated field-effect transistors with graphene on a TiO2-terminated SrTiO3 (100) crystal, and observed a large, intrinsic out-of-plane electric polarization at the hetero-interface. The measured polarization, detected by means of gate-transfer characteristics and further supported by density functional theory calculations, was found to be as large as 12 μC cm−2. The nature of the anti-hysteretic transfer characteristics in both resistance and noise is indicative of formation of trap states at the SrTiO3 interface, due to band renormalization and electrostatic confinement. These results improve the physical understanding of the interfaces between atomically thin materials and polarizable substrates.