We describe a pulsed measurement technique for suppressing hysteresis for carbon nanotube (CNT) device measurements in air, vacuum, and over a wide temperature range (80-453 K). Varying the gate ...pulse width and duty cycle probes the relaxation times associated with charge trapping near the CNT, found to be up to the 0.1-10 s range. Longer off times between voltage pulses enable consistent, hysteresis-free measurements of CNT mobility. A tunneling front model for charge trapping and relaxation is also described, suggesting trap depths up to 4-8 nm for CNTs on SiO2. Pulsed measurements will also be applicable for other nanoscale devices such as graphene, nanowires, or molecular electronics, and could enable probing trap relaxation times in a variety of material system interfaces.
We demonstrate a reliable technique for counting atomic planes (n) of few-layer graphene (FLG) on SiO2/Si substrates by Raman spectroscopy. Our approach is based on measuring the ratio of the ...integrated intensity of the G graphene peak and the optical phonon peak of Si, I(G)/I(Si), and is particularly useful in the range n > 4 where few methods exist. We compare our results with atomic force microscopy (AFM) measurements and Fresnel equation calculations. Then, we apply our method to unambiguously identify n of FLG devices on SiO2 and find that the mobility (μ ≈ 2000 cm2 V−1 s−1) is independent of layer thickness for n > 4. Our findings suggest that electrical transport in gated FLG devices is dominated by carriers near the FLG/SiO2 interface and is thus limited by the environment, even for n > 4.
Ultrathin transition metal dichalcogenides (TMDCs) have recently been extensively investigated to understand their electronic and optical properties. Here we study ultrathin Mo0.91W0.09Te2, a ...semiconducting alloy of MoTe2, using Raman, photoluminescence (PL), and optical absorption spectroscopy. Mo0.91W0.09Te2 transitions from an indirect to a direct optical band gap in the limit of monolayer thickness, exhibiting an optical gap of 1.10 eV, very close to its MoTe2 counterpart. We apply tensile strain, for the first time, to monolayer MoTe2 and Mo0.91W0.09Te2 to tune the band structure of these materials; we observe that their optical band gaps decrease by 70 meV at 2.3% uniaxial strain. The spectral widths of the PL peaks decrease with increasing strain, which we attribute to weaker exciton–phonon intervalley scattering. Furthermore, strained MoTe2 and Mo0.91W0.09Te2 extend the range of band gaps of TMDC monolayers further into the near-infrared, an important attribute for potential applications in optoelectronics.
Two-dimensional (2D) materials have recently been incorporated into resistive memory devices because of their atomically thin nature, but their switching mechanism is not yet well understood. Here we ...study bipolar switching in MoTe2-based resistive memory of varying thickness and electrode area. Using scanning thermal microscopy (SThM), we map the surface temperature of the devices under bias, revealing clear evidence of localized heating at conductive “plugs” formed during switching. The SThM measurements are correlated to electro-thermal simulations, yielding a range of plug diameters (250 to 350 nm) and temperatures at constant bias and during switching. Transmission electron microscopy images reveal these plugs result from atomic migration between electrodes, which is a thermally-activated process. However, the initial forming may be caused by defect generation or Te migration within the MoTe2. This study provides the first thermal and localized switching insights into the operation of such resistive memory and demonstrates a thermal microscopy technique that can be applied to a wide variety of traditional and emerging memory devices.
We use single-walled carbon nanotube (CNT) crossbar electrodes to probe sub-5 nm memory domains of thin AlO x films. Both metallic and semiconducting CNTs effectively switch AlO x bits between memory ...states with high and low resistance. The low-resistance state scales linearly with CNT series resistance down to ∼10 MΩ, at which point the ON-state resistance of the AlO x filament becomes the limiting factor. Dependence of switching behavior on the number of cross-points suggests a single channel to dominate the overall characteristics in multi-crossbar devices. We demonstrate ON/OFF ratios up to 5 × 105 and programming currents of 1 to 100 nA with few-volt set/reset voltages. Remarkably low reset currents enable a switching power of 10–100 nW and estimated switching energy as low as 0.1–10 fJ per bit. These results are essential for understanding the ultimate scaling limits of resistive random access memory at single-nanometer bit dimensions.
We report the development of a multilayered graphene-Al2O3 nanopore platform for the sensitive detection of DNA and DNA–protein complexes. Graphene-Al2O3 nanolaminate membranes are formed by ...sequentially depositing layers of graphene and Al2O3, with nanopores being formed in these membranes using an electron-beam sculpting process. The resulting nanopores are highly robust, exhibit low electrical noise (significantly lower than nanopores in pure graphene), are highly sensitive to electrolyte pH at low KCl concentrations (attributed to the high buffer capacity of Al2O3), and permit the electrical biasing of the embedded graphene electrode, thereby allowing for three terminal nanopore measurements. In proof-of-principle biomolecule sensing experiments, the folded and unfolded transport of single DNA molecules and RecA-coated DNA complexes could be discerned with high temporal resolution. The process described here also enables nanopore integration with new graphene-based structures, including nanoribbons and nanogaps, for single-molecule DNA sequencing and medical diagnostic applications.
Next-generation information technologies will process unprecedented amounts of loosely structured data that overwhelm existing computing systems. N3XT improves the energy efficiency of abundant-data ...applications 1,000-fold by using new logic and memory technologies, 3D integration with fine-grained connectivity, and new architectures for computation immersed in memory.
We use scanning tunneling microscopy and spectroscopy to examine the electronic nature of grain boundaries (GBs) in polycrystalline graphene grown by chemical vapor deposition (CVD) on Cu foil and ...transferred to SiO2 substrates. We find no preferential orientation angle between grains, and the GBs are continuous across graphene wrinkles and SiO2 topography. Scanning tunneling spectroscopy shows enhanced empty states tunneling conductance for most of the GBs and a shift toward more n-type behavior compared to the bulk of the graphene. We also observe standing wave patterns adjacent to GBs propagating in a zigzag direction with a decay length of ∼1 nm. Fourier analysis of these patterns indicates that backscattering and intervalley scattering are the dominant mechanisms responsible for the mobility reduction in the presence of GBs in CVD-grown graphene.
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