Quantum Carrier Reinvestment-Induced Ultrahigh and Broadband Photocurrent Responses in Graphene–Silicon Junctions

In an earlier work, we had reported a method that enables graphene–silicon junctions to display exceptionally high <i>photovoltaic</i> responses, exceeding 10⁷ V/W. Using a completely different method that has recently been reported to result in ultrahigh gain, we now show that these junctions can also demonstrate giant <i>photocurrent</i> responsivities that can approach ∼10⁷ A/W. Together, these mechanisms enable graphene–silicon junctions to be a dual-mode, broad-band, scalable, CMOS-compatible, and tunable photodetector that can operate either in photovoltage or photocurrent modes with ultrahigh responsivity values. We present detailed validation of the underlying mechanism (which we call Quantum Carrier Reinvestment, or QCR) in graphene–silicon junctions. In addition to ultrasensitive photodetection, we present QCR photocurrent spectroscopy as a tool for investigating spectral recombination dynamics at extremely low incident powers, a topic of significant importance for optoelectronic applications. We show that such spectroscopic studies can also provide a direct measure of photon energy values associated with various allowed optical transitions in silicon, again an extremely useful technique that can in principle be extended to characterize electronic levels in arbitrary semiconductors or nanomaterials. We further show the significant impact that underlying substrates can have on photocurrents, using QCR-photocurrent mapping. Contrary to expectations, QCR-photocurrents in graphene on insulating SiO₂ substrates can be much higher than its intrinsic photocurrents, and even larger than QCR-photocurrents obtained in graphene overlaying semiconducting or metallic substrates. These results showcase the vital role of substrates in photocurrent measurements in graphene or potentially in other similar materials which have relatively high carrier mobility values

Large-Area Synthesis of Graphene on Palladium and Their Raman Spectroscopy

We present a detailed investigation of the nucleation sites, growth, and morphology of large-area graphene samples synthesized via chemical vapor deposition (CVD) on bulk palladium substrates. The CVD chamber was systematically controlled over a large range of growth temperatures and durations, and the nature of graphene growth under these conditions was thoroughly investigated using a combination of scanning electron microscopy and a statistical analysis of >500 Raman spectra. Graphene growth was found to initiate at ∼825 °C, above which the growth rate increased rapidly. At <i>T</i> = 1000 °C, defect-free high-quality graphene was found to grow at an unprecedented rate of tens of micrometers per second, orders of magnitude faster than past reports on Cu- or Ni-based growth, thus leading to macroscopic coverage of the substrate within seconds of growth initiation. By arresting the growth at lower temperatures, we found that graphene nanoislands preferred to nucleate at very specific positions close to terrace edges and step inner edges. Evidence of both epitaxial and self-limiting growth was found. Along with monolayer graphene, both Bernal and turbostratic multilayer graphene could be obtained. A detailed evolution of the different types of graphene, as a function of both growth temperature and duration, has been presented. From these, optimal growth conditions for any chosen type of graphene sample can be inferred

Graphene as a Massless Electrode for Ultrahigh-Frequency Piezoelectric Nanoelectromechanical Systems

Designing “ideal electrodes” that simultaneously guarantee low mechanical damping and electrical loss as well as high electromechanical coupling in ultralow-volume piezoelectric nanomechanical structures can be considered to be a key challenge in the NEMS field. We show that mechanically transferred graphene, floating at van der Waals proximity, closely mimics “ideal electrodes” for ultrahigh frequency (0.2 GHz < <i>f</i>₀ < 2.6 GHz) piezoelectric nanoelectromechanical resonators with negligible mechanical mass and interfacial strain and perfect radio frequency electric field confinement. These unique attributes enable graphene-electrode-based piezoelectric nanoelectromechanical resonators to operate at their theoretically “unloaded” frequency-limits with significantly improved electromechanical performance compared to metal-electrode counterparts, despite their reduced volumes. This represents a spectacular trend inversion in the scaling of piezoelectric electromechanical resonators, opening up new possibilities for the implementation of nanoelectromechanical systems with unprecedented performance