ARTICLE Toward Sensitive Graphene NanoribbonÀNanopore Devices by Preventing Electron Beam-Induced Damage Matthew Puster,†,‡,§ Julio A. Rodrı´guez-Manzo,†,§ Adrian Balan,†,§ and Marija Drndic†,* †Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States and ‡Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States. §M. Puster, J. A. Rodríguez-Manzo, and A. Balan contributed equally to this work. ABSTRACT Graphene-based nanopore devices are promising candidates for next-generation DNA sequencing. Here we fabricated graphene nanoribbonÀnanopore (GNR-NP) sensors for DNA detection. Nanopores with diameters in the range 2À10 nm were formed at the edge or in the center of graphene nanoribbons (GNRs), with widths between 20 and 250 nm and lengths of 600 nm, on 40 nm thick silicon nitride (SiNx) membranes. GNR conductance was monitored in situ during electron irradiation-induced nanopore formation inside a transmission electron microscope (TEM) operating at 200 kV. We show that GNR resistance increases linearly with electron dose and that GNR conductance and mobility decrease by a factor of 10 or more when GNRs are imaged at relatively high magnification with a broad beam prior to making a nanopore. By operating the TEM in scanning TEM (STEM) mode, in which the position of the converged electron beam can be controlled with high spatial precision via automated feedback, we were able to prevent electron beam-induced damage and make nanopores in highly conducting GNR sensors. This method minimizes the exposure of the GNRs to the beam before and during nanopore formation. The resulting GNRs with unchanged resistances after nanopore formation can sustain microampere currents at low voltages (∼50 mV) in buffered electrolyte solution and exhibit high sensitivity, with a large relative change of resistance upon changes of gate voltage, similar to pristine GNRs without nanopores. KEYWORDS: DNA . sequencing . graphene nanoribbon . nanopore . silicon nitride . TEM . STEM raphene is a uniquely qualified ma- Graphene nanoribbonÀnanopore (GNR- terial for nanopore-based DNA se- NP) sensors have been proposed for G 11À14 quencing. It is one atom thin;as DNA sequencing. The mechanism sug- thin as an individual DNA base;and has gested to sequence DNA relies on measur- low electrochemical reactivity when oper- ing the modulation of current flowing ated in buffered solutions.1 As a zero- through a GNR induced by each base in an gap semimetal with high conductivity, gra- unlabeled single-stranded DNA molecule as phene exhibits an ambipolar field effect it passes through a nanopore in or next with high mobilities for both electrons and to that GNR. The atomic structure of each holes2 and is very sensitive to the local nucleotide is expected to result in a unique electrostatic potential, showing single- electrostatic potential that will modulate molecule detection at room temperature.3 the charge density in the surrounding nar- Graphene structures can be positioned and row GNR, causing a modulation of its mea- patterned via electron-beam lithography sured conductance. This effect is much like and oxygen plasma etching to form gra- the operation of a field effect transistor, phene nanoribbons (GNRs)4,5 and nano- where the potential applied to the gate * Address correspondence to [email protected]. constrictions.6,7 Alternatively, formation of modulates the charge density in the semi- graphene structures by transmission elec- conducting channel and alters the current Received for review September 29, 2013 tron beam ablation lithography (TEBAL)8 flowing through the transistor. Standard and accepted November 13, 2013. ff ∼ μ a ords the most precise control of device currents sustained by GNRs ( 1 A) are Published online width, enabling graphene structures down orders of magnitude higher than standard 10.1021/nn405112m to a few nanometers in width,9 which can ionic currents (∼0.1À10 nA) passing through 10 sustain microampere currents. nanopores; thus GNRs are expected to yield C XXXX American Chemical Society PUSTER ET AL. VOL. XXX ’ NO. XX ’ 000–000 ’ XXXX A www.acsnano.org ARTICLE Figure 1. Single-layer graphene nanoribbonÀnanopore device. (aÀc) TEM images of GNR devices. The dark gray areas are graphene covered with a 15 nm thick layer of hydrogen silsesquioxane (HSQ). Light gray areas are the bare 40 nm thick supporting silicon nitride (SiNx) membrane. (a) Nanopore formed in the center of the GNR (L = 600 nm, w = 240 nm) by converging the electron beam in TEM mode. The black circle indicates a damaged region within a radius of ∼75 nm; however, that damage is not visible in the TEM image. Inset of (a): High-magnification TEM image of the nanopore (d = 7 nm). (b) GNR (L = 600 nm, w = 85 nm) prior to nanopore drilling. (c) Nanopore (d = 4 nm) formed next to a GNR. (d) Schematic showing the GNR-NP device and the circuit diagram used for electrolytic gating in KCl solution. (e) GNR resistance (Rds) vs gate voltage (Vg) during electrolytic gating of a GNR before nanopore formation. higher signal-to-noise ratio and sequencing speeds used for nanopore formation in the TEM affect the gating with a potential to read at 10 MHz bandwidths, i.e., response of GNR-NP sensors. 10 million bases per second, for genome sequencing RESULTS AND DISCUSSION in 15 min. However, GNR-NP devices have yet to be realized. Theoretical models assume suspended GNRs Nanopores with diameters ranging from 2 to 10 nm with nanopores, precise edge structure, and no elec- were formed with a converged TEM electron beam at trolyte solution or screening.11,13,14 Practical realization the edges or in the center of 20À250 nm wide, 600 nm requires GNR-NPs to be supported on substrates, at long GNRs, fabricated in 40 nm thick silicon nitride least partially along the edges, and GNR edge structure membranes. GNRs were patterned from CVD-grown is not likely to be well-defined. DNA-induced conduc- single-layer graphene using electron-beam lithogra- tance modulations in silicon nanowires were observed phy on negative-tone resist hydrogen silsesquioxane by Xie et al.,15,16 and the model they develop predicts a (HSQ, Dow Corning XR-1541, 2%)17,18 and oxygen decay in perturbing potential with distance from the plasma etching, resulting in GNRs sandwiched be- nanopore. A calculation based on this model is in- tween the SiNx membrane and the HSQ etch mask. cluded in Figure S1. Compared to such sensors, GNRs Figure 1 shows TEM images of a 240 nm wide GNR offer the potential for better spatial resolution and with a 7 nm diameter nanopore in the center of the higher change in device conductance due to nearby GNR (Figure 1a), an 85 nm wide GNR prior to making a molecules. nanopore (Figure 1b), and a 4 nm diameter nanopore In this paper we report the fabrication and electrical formed at the edge of a GNR (Figure 1c). The GNR characterization of GNR-NP sensors (Figure 1) for bio- is visible in TEM because of the ∼15 nm thick HSQ molecule detection and analysis, measurements of protective layer serving as the etch mask that has been GNR conductance during nanopore formation, and a left on top of the graphene. The thickness of the HSQ methodology for preventing electron beam-induced etch mask was confirmed by atomic force microcopy damage to GNRs. We also show how different procedures (AFM) height measurements (Figure S2). Details of PUSTER ET AL. VOL. XXX ’ NO. XX ’ 000–000 ’ XXXX B www.acsnano.org ARTICLE the CVD-grown graphene (electron diffraction, Raman spectra, etc.) and device fabrication are given in the Methods and Supporting Information sections. Addi- tional TEM images of fabricated GNRs of different widths down to 20 nm and GNRs with side gates are shown in Figures S3, S4, and S5. To characterize the gating response in a realistic electrochemical environment, the GNR-NP devices fabricated here were immersed in 1 M KCl, and a gate voltage (Vg) was applied to a Ag/AgCl solution elec- trode while the resistance (R) of the GNR was measured (Figure 1d). The GNR sensors are most sensitive to external potentials at gate voltages where the RvsVg curve is the steepest. The GNR resistance exhibits a maximum, i.e., the charge neutrality point, at Vg ≈ 140 mV (Figure 1e). The shift in the charge neutrality point from Vg = 0 V, for undoped graphene, to 140 mV indicates that the graphene is p-doped, most likely due to doping from residues left from processing.19À21 For this device shown in Figure 1e, a perturbation of the potential near the ribbon of ∼10 mV should generate a ∼60 nA change in GNR current from a baseline current of 1.5 μA, a variation large enough for measurements at high frequency (>1 MHz). Making the nanopore with the converged electron beam is the last step in our device fabrication process. While there are other ways to make nanopores,22,23 to our knowledge this is still the best way to make the smallest nanopores.24 Alternatively, nanopores could be made prior to GNR fabrication, but this method creates other challenges of precise alignment, scalabil- ity, and nanopore filling or closing. The main challenge of creating nanopores next to GNRs using an electron beam is that, to precisely locate the position for Figure 2. In situ electrical measurement of the GNR resis- nanopore drilling relative to the GNR, it is necessary tance upon exposure to TEM and STEM imaging conditions. in situ to image some part of the device at relatively high (a) Optical image of TEM sample holder showing a chip with GNR-NP devices. Right panel: Optical image magnification and thus high current densities. The showing Au electrodes leading to a GNR-NP device on top fi 200 keV electron beam energy is high enough to of a SiNx membrane together with a magni ed SEM image In situ damage graphene,25,26 so the exposure of the GNR to of the highlighted rectangle.