Showing papers by "Mildred S. Dresselhaus published in 2019"

Two-dimensional MoS2-enabled flexible rectenna for Wi-Fi-band wireless energy harvesting.

Abstract: The mechanical and electronic properties of two-dimensional materials make them promising for use in flexible electronics1–3. Their atomic thickness and large-scale synthesis capability could enable the development of ‘smart skin’1,3–5, which could transform ordinary objects into an intelligent distributed sensor network6. However, although many important components of such a distributed electronic system have already been demonstrated (for example, transistors, sensors and memory devices based on two-dimensional materials1,2,4,7), an efficient, flexible and always-on energy-harvesting solution, which is indispensable for self-powered systems, is still missing. Electromagnetic radiation from Wi-Fi systems operating at 2.4 and 5.9 gigahertz8 is becoming increasingly ubiquitous and would be ideal to harvest for powering future distributed electronics. However, the high frequencies used for Wi-Fi communications have remained elusive to radiofrequency harvesters (that is, rectennas) made of flexible semiconductors owing to their limited transport properties9–12. Here we demonstrate an atomically thin and flexible rectenna based on a MoS2 semiconducting–metallic-phase heterojunction with a cutoff frequency of 10 gigahertz, which represents an improvement in speed of roughly one order of magnitude compared with current state-of-the-art flexible rectifiers9–12. This flexible MoS2-based rectifier operates up to the X-band8 (8 to 12 gigahertz) and covers most of the unlicensed industrial, scientific and medical radio band, including the Wi-Fi channels. By integrating the ultrafast MoS2 rectifier with a flexible Wi-Fi-band antenna, we fabricate a fully flexible and integrated rectenna that achieves wireless energy harvesting of electromagnetic radiation in the Wi-Fi band with zero external bias (battery-free). Moreover, our MoS2 rectifier acts as a flexible mixer, realizing frequency conversion beyond 10 gigahertz. This work provides a universal energy-harvesting building block that can be integrated with various flexible electronic systems. Integration of an ultrafast flexible rectifier made from a two-dimensional material with a flexible antenna achieves wireless energy harvesting of Wi-Fi radiation, which could power future flexible electronic systems.

Asymmetric hot-carrier thermalization and broadband photoresponse in graphene-2D semiconductor lateral heterojunctions

Abstract: The massless Dirac electron transport in graphene has led to a variety of unique light-matter interaction phenomena, which promise many novel optoelectronic applications. Most of the effects are only accessible by breaking the spatial symmetry, through introducing edges, p-n junctions, or heterogeneous interfaces. The recent development of direct synthesis of lateral heterostructures offers new opportunities to achieve the desired asymmetry. As a proof of concept, we study the photothermoelectric effect in an asymmetric lateral heterojunction between the Dirac semimetallic monolayer graphene and the parabolic semiconducting monolayer MoS2. Very different hot-carrier cooling mechanisms on the graphene and the MoS2 sides allow us to resolve the asymmetric thermalization pathways of photoinduced hot carriers spatially with electrostatic gate tunability. We also demonstrate the potential of graphene-2D semiconductor lateral heterojunctions as broadband infrared photodetectors. The proposed structure shows an extreme in-plane asymmetry and provides a new platform to study light-matter interactions in low-dimensional systems.

Synergistic Cobalt Sulfide/Eggshell Membrane Carbon Electrode.

Abstract: The preparation of green, facile, and cost-effective energy storage materials remains a big challenge. In this paper, a cobalt sulfide/porous carbon (Co4S3/PC) composite electrode is facilely prepared using the natural eggshell membrane (ESM) as a basal substrate. Under hydrothermal conditions, Co4S3 is grown on the ESM to form Co4S3/ESM and carbonized to form Co4S3/PC. The as-synthesized Co4S3/PC composite is used as an electrode material. The carbide from the ESM shows a porous structure and high specific surface area, which provides large space for Co4S3 attaching and ion migrating. Co4S3/PC shows much higher specific capacitance values than the sum of Co4S3 and PC electrodes, indicating a significant synergistic effect. More importantly, the Co4S3 is a typical faradic material, which exchanges Faraday charge with an electrolyte and subsequently transmits an electron to the whole electrode due to the high conductivity of the carbonized ESM. Such a synergistic effect offers the as-synthesized Co4S3/PC e...

Raman enhancement of blood constituent proteins using graphene

Abstract: Raman spectroscopy has drawn considerable attention in biomedical sensing due to the promise of label-free, multiplexed, and objective analysis along with the ability to gain molecular insights into complex biological samples. However, its true potential is yet to be realized due to the intrinsically weak Raman signal. Here, we report a simple, inexpensive and reproducible signal enhancement strategy featuring graphene as a substrate. Taking key blood constituent proteins as representative examples, we show that Raman spectra acquired from biomacromolecules can be reproducibly enhanced when these molecules are placed in contact with graphene. In particular, we demonstrate that hemoglobin and albumin display significant, but different, enhancement with the enhancement factor depending on the Raman modes, excitation wavelengths, and analyte concentrations. This technique offers a new strategy for label-free biosensing owing to the molecular fingerprinting capability, signal reliability, and simplicity of th...

Direct Observation of Symmetry-Dependent Electron-Phonon Coupling in Black Phosphorus.

Abstract: Electron–phonon coupling in two-dimensional nanomaterials plays a fundamental role in determining their physical properties. Such interplay is particularly intriguing in semiconducting black phosph...

Topological Protection Can Arise from Thermal Fluctuations and Interactions

Abstract: Topological quantum and classical materials can exhibit robust properties that are protected against disorder, for example, for noninteracting particles and linear waves. Here, we demonstrate how to construct topologically protected states that arise from the combination of strong interactions and thermal fluctuations inherent to soft materials or miniaturized mechanical structures. Specifically, we consider fluctuating lines under tension (e.g., polymer or vortex lines), subject to a class of spatially modulated substrate potentials. At equilibrium, the lines acquire a collective tilt proportional to an integer topological invariant called the Chern number. This quantized tilt is robust against substrate disorder, as verified by classical Langevin dynamics simulations. This robustness arises because excitations in this system of thermally fluctuating lines are gapped by virtue of interline interactions. We establish the topological underpinning of this pattern via a mapping that we develop between the interacting-lines system and a hitherto unexplored generalization of Thouless pumping to imaginary time. Our work points to a new class of classical topological phenomena in which the topological signature manifests itself in a structural property observed at finite temperature rather than a transport measurement.

Phonon Localization in Heat Conduction

Abstract: Nondiffusive phonon thermal transport, extensively observed in nanostructures, has largely been attributed to classical size effects, ignoring the wave nature of phonons. We report localization behavior in phonon heat conduction due to multiple scattering and interference events of broadband phonons, by measuring the thermal conductivities of GaAs/AlAs superlattices with ErAs nanodots randomly distributed at the interfaces. With an increasing number of superlattice periods, the measured thermal conductivities near room temperature increased and eventually saturated, indicating a transition from ballistic to diffusive transport. In contrast, at cryogenic temperatures the thermal conductivities first increased but then decreased, signaling phonon wave localization, as supported by atomistic Greenșs function simulations. The discovery of phonon localization suggests a new path forward for engineering phonon thermal transport.

Coexistence of Van Hove Singularities and Pseudomagnetic Fields in Modulated Graphene Bilayer

Abstract: The stacking and bending of graphene are trivial but extremely powerful agents of control over graphene's manifold physics. By changing the twist angle, one can drive the system over a plethora of exotic states via strong electron correlation, thanks to the moire superlattice potentials, while the periodic or triaxial strains induce discretization of the band structure into Landau levels without the need for an external magnetic field. We fabricated a hybrid system comprising both the stacking and bending tuning knobs. We have grown the graphene monolayers by chemical vapor deposition, using $^{12}$C and $^{13}$C precursors, which enabled us to individually address the layers through Raman spectroscopy mapping. We achieved the long-range spatial modulation by sculpturing the top layer ($^{13}$C) over uniform magnetic nanoparticles (NPs) deposited on the bottom layer ($^{12}$C). An atomic force microscopy study revealed that the top layer tends to relax into pyramidal corrugations with C$_3$ axial symmetry at the position of the NPs, which have been widely reported as a source of large pseudomagnetic fields (PMFs) in graphene monolayers. The modulated graphene bilayer (MGBL) also contains a few micrometer large domains, with the twist angle ~ 10$^{\circ}$, which were identified via extreme enhancement of the Raman intensity of the G-mode due to formation of Van Hove singularities (VHSs). We thereby conclude that the twist induced VHSs coexist with the PMFs generated in the strained pyramidal objects without mutual disturbance. The graphene bilayer modulated with magnetic NPs is a non-trivial hybrid system that accommodates features of twist induced VHSs and PMFs in environs of giant classical spins.

Double resonance raman spectroscopy of two-dimensional materials

Abstract: In this chapter, we overview double resonance Raman spectra of two dimensional materials. Many weak Raman spectral peaks are observed in the two dimensional materials which can be attributed to second order, double resonance Raman spectra. It is useful for material characterization to understand not only first order Raman spectra but also second order Raman spectra since the second order Raman spectra has more information on electronic structure of the materials than the first order Raman spectra. Combined with the conventional first order resonance Raman theory, we will explain why the double resonance condition can be strong in the two dimensional materials. Since the double resonance Raman spectra give the information of phonon with non-zero wavevectors in the Brillouin zone, both the resonant wavevector and corresponding Raman spectra can shift with changing the incident laser energy. Here we will discuss the physics of double resonance Raman spectra of graphene, transition metal dichalcogenides by theoretical analysis using the first principles calculation.

Two-dimensional MoS2-enabled flexible rectenna for wireless energy harvesting in the Wi-Fi band (Conference Presentation)

Abstract: MoS2 has attracted substantial attention due to its atomic thickness and outstanding electronic and mechanical properties. As one of the thinnest semiconductors in the world, MoS2 is promising to build flexible electronics that can be integrated with objects with arbitrary shapes and inspires a vision of distributed ubiquitous electronics. Despite recent advances in two-dimensional materials-based electronics (e.g. 2D materials-based transistors, memory devices and sensors), an efficient and flexible energy harvesting solution is necessary, but still missing, to enable a self-powered system. At the same time, the electromagnetic (EM) radiation in the Wi-Fi band (2.4 GHz and 5.9 GHz) is becoming increasingly ubiquitous and it would be beneficial to be able to wirelessly harvest it to power future distributed electronics. However, the rectennas (i.e. RF energy harvesters) based on flexible semiconductors have not been fast enough to cover the Wi-Fi band due to their limited transport properties. Here we present a unique MoS2 semiconducting-metallic phase heterojunction, which enables a flexible and high-speed Schottky diode with a cutoff frequency of 10 GHz. Due to a novel lateral architecture and self-aligned phase engineering, our MoS2 Schottky diode exhibits significantly reduced parasitic capacitance and series resistance. By integrating the MoS2 rectifier with a flexible Wi-Fi band antenna, we successfully fabricate a fully flexible rectenna that demonstrates direct energy harvesting of EM radiation in the Wi-Fi band with zero external bias (battery-free). Moreover, taking advantage of the nonlinearity of the MoS2 Schottky diode, a frequency mixing in the gigahertz range is also successfully demonstrated on flexible substrates.