Session A16: 2D Devices: Sensors and Detectors

On-Chip High-Responsivity Graphene$-$Boron Nitride Photodetector Integrated with Si Waveguide

Graphene and other two-dimensional (2D) materials have emerged as promising materials for broadband and high-speed photodetection [1] and optical modulation [2]. These optoelectronic capabilities can augment complementary metal$-$oxide$-$ semiconductor (CMOS) devices for high-speed and low-power optical interconnects. Here, we demonstrate an on-chip ultrafast photodetector based on a two-dimensional heterostructure consisting of high-quality graphene encapsulated in hexagonal boron nitride. Coupled to the optical mode of a silicon waveguide, this 2D heterostructure-based photodetector exhibits a maximum responsivity of 0.36 A/W and high-speed operation with a 3 dB cutoff at 42 GHz [3]. From photocurrent measurements as a function of the top-gate and source-drain voltages, we conclude that the photoresponse is consistent with hot electron mediated effects. 1. Gan, X. et al. Chip-integrated ultrafast graphene photodetector with high responsivity. Nat. Photonics 7, 883--887 (2013). 2. Gao, Y. et al. High-Speed Electro-Optic Modulator Integrated with Graphene-Boron Nitride Heterostructure and Photonic Crystal Nanocavity. Nano Lett. 15(3), 2001-2005 (2015) 3. Shiue, R.-J. et al. High-Responsivity Graphene--Boron Nitride Photodetector and Autocorrelator in a Silicon Photonic Integrated Circuit. Nano Lett. Article ASPA   

Buckled Graphene-like Materials in Ultrafast and Superstrong Optical Fields

We discuss the theoretical investigation of buckled Dirac materials (silicene and germanene) interacting with ultrashort and ultrastrong optical pulses. Highly intensive few-cycle fields strongly modify the electronic and optical properties of these two dimensional materials. The strong nonlinearity of the system for the fields applied (~V/{\AA}), will cause the violation of the charge (C) and parity (P) and time reversal symmetries. Such symmetry violations are related to the electron transfer between the sublattices produced by the normal field component and result in nonreciprocity, optical rectification and the appearance of a cross current. We also note a direct resemblance between silicene and field-effect transistors (FET). In FETs, the (perpendicular) gate field changes the carrier concentration and thereby, controls its conductance. Analogously, in silicene, the normal field component of the pulse, transfers carriers between two sublattices, and consequently modulates the response function of silicene to the in-plane field. \textbf{Reference:} H. K. Kelardeh, V. Apalkov, and M. I. Stockman, \textit{Ultrafast field control of symmetry, reciprocity, and reversibility in buckled graphene-like materials}, Phys. Rev. B \textbf{92} (4), 045413 (2015).   

Optical Detection of Local Electric Field Dynamics in Solutions by Waveguide-integrated Graphene Device

The spatio-temporal dynamics of local electric fields in ionic solutions plays a central role in various chemical and biological processes ranging from batteries technologies to neuron signaling. A non-invasive, precise detection scheme for measuring local electric fields dynamics has long been sought for. Here, we report a sensitive, high-speed, high spatial resolution optical imaging method for local electric fields based on the unique optoelectronic properties of graphene. With enhancement from a waveguide involving critical coupling concept, we show that our graphene optical sensor provides an ideal platform for studying dynamics of local electric field fluctuations in different nonequilibrium solutions.~   

Passive optical switches based on endohedral fullerenes

Although there have been many attempts to find better nanomaterial-based optical limiters {\&} switches in recent years, currently there are only a few effective options for high-energy lasers. Reverse saturable absorption in fullerenes has been widely used to realize excellent passive optical limiters for the visible region up to 650 nm. The electronic structure of fullerenes can be modified by the encapsulation of endohedral clusters to achieve exotic quantum states of matter such as superconductivity. Building on this concept, in this talk, we show that three tri-metallic nitride endohedral fullerenes could alter the HOMO-LUMO gap and allow passive optical switching with a low limiting threshold (0.3 J/cm2) and a wider operation window up to 1064 nm (average pulse energy\textgreater 0.5 mJ in ns regime).   

Kirigami Graphene Transistors for Biological Sensing

As flexible, locally amplifying probes, graphene transistors have potential applications in biological sensing, particularly for read-out of extracellular potentials. We present here electrolyte-gating measurements of stretchable graphene transistors aimed at exploring this application. The graphene is etched into patterns inspired by the Japanese paper art of kirigami to permit in-plane stretching \footnote{Blees, M. K. et al. Graphene kirigami. Nature (2015).}. Using a technique developed in our group for manipulating these devices in solution, we can maneuver and stretch devices in an electrolyte solution while monitoring their electrical response. These devices show proximity-dependent gating to voltages on an additional small metal probe near the device, and we quantify the nature and sensitivity of this response. The flexibility of these devices makes them promising as “wearable” electronics for cells, and we present early results on interactions between graphene devices and cardiomyocyte cells.   

Nanopores in suspended WS$_2$ membranes for DNA sequencing

Recent advances in solid-state nanopore sensor systems for DNA detection and analysis have been supported by using increasingly thinner materials to the point of utilizing atomically thin two-dimensional materials such as graphene and MoS$_2$. However, these materials still have issues with pore wettability and signal-to-noise ratios displayed in DNA translocation measurements. Recently, the fabrication and operation of nanopores in MoS$_2$ have been demonstrated, but the wetting properties and signal-to-noise ratios of transition metal dichalcogenides are yet to be understood and further improved. Here we fabricate suspended WS$_2$ nanopore devices with sub-10 nm pore diameters using a novel nanomaterial transfer method and TEM nanosculpting to study and better understand nanopore wetting properties and performance in DNA translocation measurements.   

Electronic nanobiosensors based on two-dimensional materials

Atomically-thick two-dimensional (2D) nanomaterials have tremendous potential to be applied as transduction elements in biosensors and bioelectronics. We developed scalable methods for synthesis and large-area transfer of two-dimensional nanomaterials, particularly graphene and metal dichalcogenides (so called ``MX$_{\mathrm{2}}$'' materials). We also developed versatile fabrication methods for large arrays of field-effect transistors (FETs) and micro-electrodes with these nanomaterials based on either conventional photolithography or innovative approaches that minimize contamination of the 2D layer. By functionalizing the FETs with a computationally redesigned water-soluble mu-opioid receptor, we created selective and sensitive biosensors suitable for detection of the drug target naltrexone and the neuropeptide enkephalin at pg/mL concentrations. We also constructed DNA-functionalized biosensors and nano-particle decorated biosensors by applying related bio-nano integration techniques. Our methodology paves the way for multiplexed nanosensor arrays with all-electronic readout suitable for inexpensive point-of-care diagnostics, drug-development and biomedical research. With graphene field-effect transistors, we investigated the graphene/solution interface and developed a quantitative model for the effect of ionic screening on the graphene carrier density based on theories of the electric double layer. Finally, we have developed a technique for measuring low-level Faradaic charge-transfer current (fA) across the graphene/solution interface via real-time charge monitoring of graphene microelectrodes in ionic solution. This technique enables the development of flexible and transparent pH sensors that are promising for \textit{in vivo} applications.   

\textbf{Enhancing the performance of Graphene NEMS}

In recent past Nanoelectromechanical systems (NEMS) have got several sensing based applications such as force, spin, charge and mass sensors. These devices due to their smaller size, operate in very high frequency regime (MHz - GHz) and have very high quality factors (10$^{2}$ -10$^{5})$. Nevertheless these devices are limited by their comparatively smaller linear operational range. Electromechanical devices based on 2D materials are extremely sensitive to strain. We studied the effect of strain on the performance of single layer Graphene NEMS. Our results reveal that the strain in Graphene NEMS can be tuned to increase the linear operational range. We report a 25 dB increase in dynamic range by tuning the strain from 10$^{-3\, }$at room temperature to 10$^{-2}$ at 200K. This increase in dynamic range is also accompanied by partial cancellation of elastic and electrostatic nonlinearities. The resulting mass resolution estimated from the experimental data is 100 yg...................$^{1}$ which is one order of magnitude better than previously reported values. Reference: 1. Parmar, M. M., Gangavarapu, P. R. Y. {\&} Naik, A. K. Dynamic range tuning of graphene nanoresonators. \textit{Applied Physics Letters} \textbf{107,} 113108 (2015).   

Biosensors based on DNA-Functionalized Graphene

Since its discovery, graphene has been used for sensing applications due to its outstanding electrical properties and biocompatibility. Here, we demonstrate the capabilities of~field effect transistors (FETs)~based on~CVD-grown graphene functionalized with commercially obtained DNA oligomers and~aptamers~for detection of various biomolecular targets (e.g., complementary DNA and small molecule drug targets). Graphene~FETs were created with a scalable photolithography process that produces arrays consisting of~50-100~FETs with a layout~suitable for multiplexed detection of four molecular targets. FETs were characterized via AFM to confirm the presence of the aptamer.~From the measured electrical characteristics, it was determined that binding of molecular targets by the DNA chemical recognition element led to a reproducible, concentration-dependent shift in the Dirac voltage. This biosensor class is potentially suitable for applications in drug~detection. This work is funded by NIH through the Center for AIDS Research at the University of Pennsylvania.   

Direct observation of PMMA removal from graphene surface

PMMA is often used as a carrier layer for transfer of CVD graphene from copper to other substrates. After transfer, the PMMA is removed by chemical or thermal treatment. However, regardless of the method used, polymer residues are left on the graphene surface, which degrade the performance of graphene-based devices. Here, we present a systematic study of PMMA removal after graphene transfer. Raman and FET measurements were applied to monitor the polymer dissolution in an acetone bath. Isotope labeling and in-situ TOF-SIMS, XPS, Raman and AFM all show chemical changes in surface residues upon vacuum annealing. These data along with strategies to clean the graphene surface will be presented.   

Carbon nanotube as a local drag sensor

We report a Columb drag measurement in a carbon nanotube (CNT) and graphene hybrid device. In this device, the CNT and graphene serve as a 1D and 2D electronic system respectively and are separated by a thin hexagonal boron nitride (h-BN). By flowing a drive current in one conductor, due to electron-electron interactions, a drag voltage is developed in the other conductor. In the case where a current is applied to graphene, the CNT can act as a local drag sensor that probes the microscopic effects of electronic interactions hidden in graphene. We demonstrate this drag sensor capability by applying a magnetic field in graphene and show the transition from compressible states to incompressible states.   

Amino Acids Interaction with Boron Nitride Nanomaterials

Stability and electronic properties of bioconjugated BN nanostructures are investigated. The biomolecules considered are amino acids ranging from monomers to peptides, and the low- dimensional BN nanostructures are monolayer and tubular configurations. Specifically, we examine the role of an aqueous background in stabilizing such bioconjugated nanostructures by employing the continuum solvent model. It is expected that the calculated results will provide guidance for selective sensing of biomolecules by the next generation nanomaterials.   

Biomolecular interactions of emerging two-dimensional materials with aromatic amino acids .

The present work experimentally investigates the interaction of aromatic amino acids, viz., tyrosine, tryptophan, and phenylalanine with novel two-dimensional (2D) materials including graphene (G), graphene oxide (GO), and boron nitride (BN). Photoluminescence, micro-Raman spectroscopy and cyclic voltammetry were employed to investigate the nature of interactions and possible charge transfer between 2D materials and amino acids. Consistent with previous theoretical studies $^{\mathrm{[1,2]}}$, graphene and BN were observed to interact with amino acids through $\pi $-$\pi $ interactions. Furthermore, we found that GO exhibits strong interactions with tryptophan and tyrosine as compared to graphene and BN, which we attribute to the formation of H-bonds between tryptophan and GO as shown theoretically in Ref. 2. On the other hand, phenylalanine did not exhibit much difference in interactions with G, GO, and BN.(1) The Journal of Chemical Physics 130, 124911 (2009) (2) J. Phys. Chem. Lett. 2013, 4, 3710$-$3718