Detection of Individual Gas Molecules Absorbed on Graphene

TL;DR

Graphene is an exceptionally low-noise material electronically, which makes it a promising candidate not only for chemical detectors but also for other applications where local probes sensitive to external charge, magnetic field or mechanical strain are required.

Abstract

The ultimate aspiration of any detection method is to achieve such a level of sensitivity that individual quanta of a measured value can be resolved. In the case of chemical sensors, the quantum is one atom or molecule. Such resolution has so far been beyond the reach of any detection technique, including solid-state gas sensors hailed for their exceptional sensitivity. The fundamental reason limiting the resolution of such sensors is fluctuations due to thermal motion of charges and defects which lead to intrinsic noise exceeding the sought-after signal from individual molecules, usually by many orders of magnitude. Here we show that micrometre-size sensors made from graphene are capable of detecting individual events when a gas molecule attaches to or detaches from graphenes surface. The adsorbed molecules change the local carrier concentration in graphene one by one electron, which leads to step-like changes in resistance. The achieved sensitivity is due to the fact that graphene is an exceptionally low-noise material electronically, which makes it a promising candidate not only for chemical detectors but also for other applications where local probes sensitive to external charge, magnetic field or mechanical strain are required.

Figures (6)

• Figure 1: Sensitivity of graphene to chemical doping. (a) Concentration$\Delta n$ of chemically-induced charge carriers in single-layer graphene exposed to different concentrations C of $\mathrm{NO}_{2}$. Upper inset: scanning-electron micrograph of this device (in false colours matching those seen in visible optics). The scale of the micrograph is given by the width of the Hall bar, which is $1 \mu \mathrm{~m}$. Lower inset: Characterisation of the graphene device by using the electric field effect. By applying positive (negative) $V_{g}$ between the Si wafer and graphene, we induced electrons (holes) in graphene in concentrations $n=\alpha \cdot V_{\mathrm{g}}$. The coefficient $\alpha \approx 7.2 \cdot 10^{10} \mathrm{~cm}^{-2} / \mathrm{N}$ was found from Hall effect measurements $[6-9]$. To measure Hall resistivity $\rho_{\mathrm{xy}}, B=1 \mathrm{~T}$ was applied perpendicular to graphene's surface. (b) - Changes in resistivity $\rho$ at zero $B$ caused by graphene's exposure to various gases diluted in concentration 1 ppm . The positive (negative) sign of changes is chosen here to indicate electron (hole) doping. Region I the device is in vacuum prior to its exposure; II - exposure to a 5 litre volume of a diluted chemical; III - evacuation of the experimental setup; and IV - annealing at $150^{\circ} \mathrm{C}$. The response time was limited by our gas-handling system and a several-second delay in our lock-in based measurements. Note that the annealing caused an initial spikelike response in $\rho$, which lasted for a few minutes and was generally irreproducible. For clarity, this transient region between III and IV is omitted, as indicated in the figure.
• Figure 2: Constant mobility of charge carriers in graphene with increasing chemical doping. Conductivity$\sigma$ of single-layer graphene away from the neutrality point changes approximately linearly with increasing $V_{g}$ and the steepness of $\sigma\left(V_{\mathrm{g}}\right)$-curves (away from the NP) characterizes mobility $\mu$ [6-9]. Doping with $\mathrm{NO}_{2}$ adds holes but also induces charged impurities. The latter apparently do not affect the mobility of either electrons or holes. The parallel shift implies a negligible scattering effect of the charged impurities induced by chemical doping. The open symbols on the curves indicate the same total concentration of holes $n_{\mathrm{t}}$ as found from Hall measurements. The practically constant $\sigma$ for the same $n_{\mathrm{t}}$ yields that the absolute mobility $\mu=\sigma / n_{t}$ e as well as the Hall mobility are unaffected by chemical doping. For further analysis and discussions, see Supplementary Information.
• Figure 3: Single-molecule detection. (a) - examples of changes in Hall resistivity observed near the neutrality point ($|n|<10^{11} \mathrm{~cm}^{-2}$ ) during adsorption of strongly diluted $\mathrm{NO}_{2}$ (blue curve) and its desorption in vacuum at 50 C (red). The green curve is a reference - the same device thoroughly annealed and then exposed to pure He. The curves are for a 3 -layer device in $B=10 \mathrm{~T}$. The grid lines correspond to changes in $\rho_{\mathrm{xy}}$ caused by adding one electron charge $e(\delta R \approx 2.5 \mathrm{Ohm})$, as calibrated in independent measurements by varying $V_{\mathrm{g}}$. For the blue curve, the device was exposed to 1 ppm of $\mathrm{NO}_{2}$ leaking at a rate of $\approx 10^{-3} \mathrm{mbar} \cdot / / \mathrm{s}$. ( $\mathbf{b}, \mathbf{c}$ ) - Statistical distribution of step heights $\delta R$ in this device without its exposure to $\mathrm{NO}_{2}$ (in helium) (b) and during a slow desorption of $\mathrm{NO}_{2}$ (c). For this analysis, all changes in $\rho_{\mathrm{xy}}$ larger than 0.5 Ohm and quicker than 10s (lock-in time constant was 1s making the response time of $\approx 6 \mathrm{~s}$ ) were recorded as individual steps. The dotted curves are automated Gaussian fits (see Supplementary Information).
• Figure 4: Figure S1. Statistical distribution of step heights $\delta R$ in a (5-7)-layer device during its exposure to pure helium (a) and a small leak ( $10^{-3} \mathrm{mbar} \cdot 1 / \mathrm{s}$ ) of $\mathrm{NO}_{2}$ diluted in a concentration of 1 ppm (b). An example of the raw data is shown by the blue curve in Fig. 3a. Red and blue bars indicate steps in the opposite directions (desorption and adsorption events, respectively). The histogram in (a) was first fitted by a Gaussian curve (green). Then, assuming that the noise peak does not change, the remaining statistical distribution was fitted by 4 Gaussian curves (black) allowing all four amplitudes and positions to be chosen automatically by the Origin-7.0 fitting routine. The resulting total of 5 Gaussians accurately fits the whole distribution (grey curve). Three Gaussians also give a reasonable (but less perfect) fit with extra peaks centred at $\pm 0.05 \mathrm{Ohm}$.
• Figure 5: Figure S2. Accumulation of dopants on graphene. Changes in the longitudinal ( $\rho_{\mathrm{xx}}$ ) and Hall ( $\rho_{\mathrm{xy}}$ ) resistivity of graphene exposed to a continuous supply of stronglydiluted $\mathrm{NH}_{3}$ (right part). After the exposure, the device was annealed close to the pristine state and then exposed to $\mathrm{NO}_{2}$ in exactly the same fashion (left part). Here, measurements of both $\rho_{\mathrm{xx}}$ and $\rho_{\mathrm{xy}}$ were carried out in field $B=1 \mathrm{~T}$.