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Detection of individual gas moleculesadsorbed on graphene

F. SCHEDIN1, A. K. GEIM1, S. V. MOROZOV2, E. W. HILL1, P. BLAKE1, M. I. KATSNELSON3

AND K. S. NOVOSELOV1*1Manchester Centre for Mesoscience and Nanotechnology, University of Manchester, Manchester, M13 9PL, UK2Institute for Microelectronics Technology, 142432 Chernogolovka, Russia3Institute for Molecules and Materials, University of Nijmegen, 6525 ED Nijmegen, Netherlands*e-mail: Konstantin.Novoselov@manchester.ac.uk

Published online: 29 July 2007; doi:10.1038/nmat1967

The ultimate aim of any detection method is to achieve sucha level of sensitivity that individual quanta of a measuredentity can be resolved. In the case of chemical sensors, thequantum is one atom or molecule. Such resolution has so farbeen beyond the reach of any detection technique, includingsolid-state gas sensors hailed for their exceptional sensitivity14.The fundamental reason limiting the resolution of such sensorsis fluctuations due to thermal motion of charges and defects5,which lead to intrinsic noise exceeding the sought-after signalfrom individualmolecules, usually bymany orders ofmagnitude.Here, we show thatmicrometre-size sensorsmade fromgrapheneare capable of detecting individual events when a gas moleculeattaches to or detaches from graphenes surface. The adsorbedmolecules change the local carrier concentration in graphene oneby one electron, which leads to step-like changes in resistance.The achieved sensitivity is due to the fact that graphene is anexceptionally low-noise material electronically, which makes ita promising candidate not only for chemical detectors but alsofor other applications where local probes sensitive to externalcharge, magnetic field or mechanical strain are required.

Solid-state gas sensors are renowned for their high sensitivity,whichin combination with low production costs and miniaturesizeshave made them ubiquitous and widely used in manyapplications1,2. Recently, a new generation of gas sensors hasbeen demonstrated using carbon nanotubes and semiconductornanowires (see, for example, refs 3,4). The high acclaim receivedby the latter materials is, to a large extent, due to their exceptionalsensitivity allowing detection of toxic gases in concentrationsas small as 1 part per billion (p.p.b.). This and even higherlevels of sensitivity are sought for industrial, environmental andmilitary monitoring.

The operational principle of graphene devices described belowis based on changes in their electrical conductivity, , due to gasmolecules adsorbed on graphenes surface and acting as donorsor acceptors, similar to other solid-state sensors14. However, thefollowing characteristics of graphene make it possible to increasethe sensitivity to its ultimate limit and detect individual dopants.First, graphene is a strictly two-dimensional material and, assuch, has its whole volume exposed to surface adsorbates, whichmaximizes their effect. Second, graphene is highly conductive,exhibitingmetallic conductivity and, hence, low Johnson noise evenin the limit of no charge carriers69, where a few extra electrons

can cause notable relative changes in carrier concentration, n.Third, graphene has few crystal defects610, which ensures a lowlevel of excess (1/f ) noise caused by their thermal switching5.Fourth, graphene allows four-probe measurements on a single-crystal device with electrical contacts that are ohmic and havelow resistance. All of these features contribute to make a uniquecombination that maximizes the signal-to-noise ratio to a levelsufficient for detecting changes in a local concentration by less thanone electron charge, e, at room temperature.

The studied graphene devices were prepared bymicromechanical cleavage of graphite at the surface of oxidizedSi wafers7. This allowed us to obtain graphene monocrystals oftypically 10 m in size. By using electron-beam lithography, wemade electrical (Au/Ti) contacts to graphene and then definedmultiterminal Hall bars by etching graphene in an oxygenplasma. The microfabricated devices (Fig. 1a, upper inset) wereplaced in a variable temperature insert inside a superconductingmagnet and characterized by using field-effect measurementsat temperatures, T , from 4 to 400K and in magnetic fields, B,up to 12 T. This allowed us to find the mobility, , of chargecarriers (typically, 5,000 cm2 V1 s1) and distinguish betweensingle-, bi- and few-layer devices, in addition to complementarymeasurements of their thickness carried out by optical and atomicforce microscopy69. Figure 1a, lower inset, shows an exampleof the field-effect behaviour exhibited by our devices at roomtemperature. This plot shows that longitudinal (xx) and Hall(xy) resistivities are symmetric and antisymmetric functionsof gate voltage, Vg, respectively. xx exhibits a peak at zero Vg,whereas xy simultaneously passes through zero, which shows thatthe transition from electron to hole transport occurs at zero Vgindicating that graphene is in its pristine undoped state6.

To assess the effect of gaseous chemicals on graphene devices,the insert was evacuated and then connected to a relatively large(5 l) glass volume containing a selected chemical strongly dilutedin pure helium or nitrogen at atmospheric pressure. Figure 1bshows the response of zero-field resistivity, = xx(B= 0)= 1/,to NO2, NH3, H2O and CO in concentrations, C, of 1 part permillion (p.p.m.). Large easily detectable changes that occurredwithin 1min and, for the case of NO2, practically immediately afterletting the chemicals in can be seen. The initial rapid responsewas followed by a region of saturation, in which the resistivitychanged relatively slowly. We attribute this region to redistribution

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0.1 1C (p.p.m.)

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Figure 1 Sensitivity of graphene to chemical doping. a, Concentration, 1n, ofchemically induced charge carriers in single-layer graphene exposed to differentconcentrations, C, of NO2. Upper inset: Scanning electron micrograph of this device(in false colours matching those seen in visible optics). The scale of the micrographis given by the width of the Hall bar, which is 1 m. Lower inset: Characterization ofthe graphene device by using the electric-field effect. By applying positive (negative)Vg between the Si wafer and graphene, we induced electrons (holes) in graphene inconcentrations n= Vg. The coefficient 7.21010 cm2 V1 was found fromHall-effect measurements69. To measure Hall resistivity, xy , B= 1 T was appliedperpendicular to graphenes surface. b, Changes in resistivity, , at zero B causedby graphenes exposure to various gases diluted in concentration to 1 p.p.m. Thepositive (negative) sign of changes is chosen here to indicate electron (hole) doping.Region I: the device is in vacuum before its exposure; II: exposure to a 5 l volume ofa diluted chemical; III: evacuation of the experimental set-up; and IV: annealing at150 C. The response time was limited by our gas-handling system and aseveral-second delay in our lock-in-based measurements. Note that the annealingcaused an initial spike-like response in , which lasted for a few minutes and wasgenerally irreproducible. For clarity, this transient region between III and IVis omitted.

of adsorbed gas molecules between different surfaces in the insert.After a near-equilibrium state was reached, we evacuated thecontainer again, which led only to small and slow changes in (region III in Fig. 1b), indicating that adsorbed molecules were

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Figure 2 Constant mobility of charge carriers in graphene with increasingchemical doping. Doping increased from zero (black curve) to1.51012 cm2(red curve) due to increasing exposure to NO2. Conductivity, , of single-layergraphene away from the neutrality point changes approximately linearly withincreasing Vg and the steepness of the (Vg ) curves (away from the neutrality point)characterizes the mobility, (refs 69). Doping with NO2 adds holes but alsoinduces charged impurities. The latter apparently do not affect the mobility of eitherelectrons or holes. The parallel shift implies a negligible scattering effect of thecharged impurities induced by chemical doping. The open symbols on the curvesindicate the same total concentration of holes, nt (2.71012 cm2), as foundfrom Hall measurements. The practically constant for the same nt yieldsthat the absolute mobility, = /nte, as well as the Hall mobility areunaffected by chemical doping. For further analysis and discussions, see theSupplementary Information.

strongly attached to the graphene devices at room temperature.Nevertheless, we found that the initial undoped state could berecovered by annealing at 150 C in vacuum (region IV). Repetitiveexposureannealing cycles showed no poisoning effects of thesechemicals (that is, the devices could be annealed back to their initialstate). A short-time ultraviolet illumination offered an alternativeto thermal annealing.

To gain further information about the observed chemicalresponse, we simultaneously measured changes in xx and xycaused by gas exposure, which allowed us to find directly (1)concentrations, 1n, of chemically induced charge carriers, (2)their sign and (3) mobilities. The Hall measurements revealed thatNO2, H2O and iodine acted as acceptors, whereas NH3, CO andethanol were donors. We also found that, under the same exposureconditions, 1n depended linearly on the concentration, C, of anexamined chemical (see Fig. 1a). To achieve the linear conductanceresponse, we electrically biased our devices (by more than 10V)to higher-concentration regions, away from the neutrality point, sothat both =ne andHall conductivity, xy=1/xy=ne/B, wereproportional to n (see Fig. 1a, lower inset)69. The linear responseas a function of C should greatly simplify the use of graphene-basedsensors in practical terms.

Chemical doping also induced impurities in graphene inconcentrations Ni = 1n. However, despite these extra scatterers,we found no notable changes in even for Ni exceeding1012 cm2. Figure 2 shows this unexpected observation by showingthe electric-field effect in a device repeatedly doped withNO2. V-shaped (Vg) curves characteristic for graphene69

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Figure 3 Single-molecule detection. a, Examples of changes in Hall resistivity observed near the neutrality point (|n|< 1011 cm2) during adsorption of strongly diluted NO2(blue curve) and its desorption in vacuum at 50 C (red curve). The green curve is a referencethe same device thoroughly annealed and then exposed to pure He. Thecurves are for a three-layer device in B= 10 T. The grid lines correspond to changes in xy caused by adding one electron charge, e (R 2.5), as calibrated inindependent measurements by varying Vg. For the blue curve, the device was exposed to 1 p.p.m. of NO2 leaking at a rate of103 mbar l s1. b,c, Statistical distribution ofstep heights, R, in this device without its exposure to NO2 (in helium) (b) and during a slow desorption of NO2 (c). For this analysis, all changes in xy larger than 0.5 andquicker than 10 s (lock-in time constant was 1 s making the response time of6 s) were recorded as individual steps. The dotted curves in textbfc are automated gaussianfits (see the Supplementary Information).

can be seen. Their slopes away from the neutrality point providea measure of impurity scattering (so-called field-effect mobility,=1/1ne =1/e1Vg). The chemical doping only shiftedthe curves as a whole, without any significant changes in theirshape, except for the fact that the curves became broader around theneutrality point (the latter effect is discussed in the SupplementaryInformation). The parallel shift unambiguously proves that thechemical doping did not affect scattering rates. Complementarymeasurements in magnetic field showed that the Hall-effectmobility, = xy/xxB, was also unaffected by the dopingand exhibited values very close to those determined from theelectric-field effect. Further analysis yields that chemically inducedionized impurities in graphene in concentrations >1012 cm2 (thatis, less than 10 nm apart) should not be a limiting factor for untilit reaches values of the order of 105 cm2 V1 s1, which translatesinto a mean free path as large as 1 m (see the SupplementaryInformation). This is in striking contrast with conventionaltwo-dimensional systems, in which such high densities of chargedimpurities are detrimental for ballistic transport, and also disagreesby a factor of >10 with recent theoretical estimates for thecase of graphene1113. Our observations clearly raise doubts aboutcharged impurities being the scatterers that currently limit ingraphene1113. In the Supplementary Information, we show that afew-nanometre-thick layer of absorbed water provides sufficientdielectric screening to explain the suppressed scattering on chargedimpurities. We also suggest there that microscopic corrugations ofa graphene sheet14,15 could be dominant scatterers.

The detection limit for solid-state gas sensors is usually definedas the minimal concentration that causes a signal exceedingthe sensors intrinsic noise14. In this respect, a typical noiselevel in our devices, 1/ 104 (see Fig. 1b), translates intothe detection limit of the order of 1 p.p.b. This already putsgraphene on par with other materials used for most sensitive gassensors14. Furthermore, to demonstrate the fundamental limitfor the sensitivity of graphene-based gas sensors, we optimizedour devices and measurements as described in the SupplementaryInformation. In brief, we used high driving currents to suppress theJohnson noise, annealed devices close to the neutrality point, whererelative changes in n were largest for the same amount of chemical

doping, and used few-layer graphene (typically, 35 layers), whichallowed a contact resistance of100, much lower than for single-layer graphene. We also used the Hall geometry that provided thelargest response to small changes in n near the neutrality point(see Fig. 1a, lower inset). In addition, this measurement geometryminimizes the sensitive area to the central region of the Hall cross(1 m2 in size) and allows changes in xy to be calibrated directlyin terms of charge transfer by comparing the chemically inducedsignal with the known response to Vg. The latter is important forthe low-concentration region, where the response of xy to changesin n is steepest, but there is no simple relation between xy and n.

Figure 3 shows changes in xy caused by adsorption anddesorption of individual gas molecules. In these experiments, wefirst annealed our devices close to the pristine state and thenexposed them to a small leak of strongly diluted NO2, which wasadjusted so that xy remained nearly constant over several minutes(that is, we tuned the system close to thermal equilibrium wherethe number of adsorption and desorption events within the Hallcross area was reasonably small). In this regime, the chemicallyinduced changes in xy were no longer smooth but occurred ina step-like manner as shown in Fig. 3a (blue curve). If we closedthe leak and started to evacuate the sample space, similar stepsoccurred but predominantly in the opposite direction (red curve).For finer control of the adsorption/desorption rates, we found ituseful to slightly adjust the temperature while keeping the sameleak rate. The characteristic size, R, of the observed steps interms of ohms depended on B, the number of graphene layersand, also, varied strongly from one device to another, reflecting thefact that the steepness of the xy curves near the neutrality point(see Fig. 1a, lower inset) could be different for different devices69.However, when the steps were recalibrated in terms of equivalentchanges in Vg, we found that to achieve the typical value of Rit always required exactly the same voltage changes, 1.5mV, forall of our 1 m devices and independently of B. The latter valuecorresponds to 1n 108 cm2 and translates into one electroncharge, e, removed from or added to the area of 1 1 m2 ofthe Hall cross (note that changes in xy as a function of Vg weresmooth, that is, no charge quantization in the devices transportcharacteristics occurredas expected). As a reference, we repeated

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the same measurements for devices annealed for 2 days at 150 Cand found no or very few steps (green curve).

The curves shown in Fig. 3a clearly suggest individualadsorption and desorption events but statistical analysis is requiredto prove this. To this end, we recorded a large number of curvessuch as that in Fig. 3a (100 h of continuous recording). Theresulting histograms with and without exposure to NO2 are shownin Fig. 3b,c (a histogram for another device is shown in theSupplementary Information). The reference curves exhibited manysmall (positive and negative) steps, which gave rise to a noisepeak at small R. Large steps were rare. On the contrary, slowadsorption of NO2 or its subsequent desorption led to manylarge, single-electron steps. The steps were not equal in size,as expected, because gas molecules could be adsorbed anywhereincluding the fringes of the sensitive area, which should result invarying contributions. Moreover, because of a finite time constant(1 s) used in these sensitive measurements, random resistancefluctuations could overlap with individual steps either enhancingor reducing them and, also, different events could overlap in timeoccasionally (such as the largest step on the red curve in Fig. 3a,which has a quadruple height). The corresponding histogram(Fig. 3c) shows the same noise peak as the reference in Fig. 3bbut, in addition, there are two extra maxima that are centredat a value of R, which corresponds to removing/adding oneacceptor from the detection area. The asymmetry in the statisticaldistribution in Fig. 3c corresponds to the fact that single-acceptorsteps occur predominantly in one direction, that is, NO2 on-averagedesorbs from graphenes surface in this particular experiment. Theobserved behaviour leaves no doubt that the changes in grapheneconductivity during chemical exposure were quantized, with eachevent signalling adsorption or desorption of a single NO2 molecule.

In summary, graphene-based gas sensors allow the ultimatesensitivity such that the adsorption of individual gas moleculescould be detected. Large arrays of such sensors would increasethe catchment area16, allowing higher sensitivity for short-timeexposures and the detection of active (toxic) gases in as minuteconcentrations as practically desirable. The epitaxial growth of few-layer graphene17,18 offers a realistic promise of mass productionof such devices. Our experiments also show that graphene issufficiently electronically quiet to be used in single-electrondetectors operational at room temperature19 and in ultrasensitivesensors of magnetic field or mechanical strain20, in which theresolution is often limited by 1/f noise. Equally important21,22 is thedemonstrated possibility of chemical doping of graphene by bothelectrons and holes in high concentrations without deterioration ofits mobility. This should allow microfabrication of pn junctions,which attract significant interest from the point of view of bothfundamental physics and applications. Despite its short history,

graphene is considered to be a promising material for electronics byboth academic and industrial researchers6,17,22, and the possibilityof its chemical doping further improves the prospects of graphene-based electronics.

Received 14 May 2007; accepted 2 July 2007; published 29 July 2007.

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AcknowledgementsWe thank A. MacDonald, S. Das Sarma and V. Falko for illuminating discussions. This work wassupported by the EPSRC (UK) and the Royal Society. M.I.K. acknowledges financial support fromFOM (Netherlands).Correspondence and requests for materials should be addressed to K.S.N.Supplementary Information accompanies this paper on www.nature.com/naturematerials.

Author contributionsK.S.N. designed the experiment and carried out both experimental work and data analysis, A.K.G.suggested the research direction and wrote the manuscript, F.S. and P.B. made graphene devices,S.V.M. and E.W.H. helped with experiments and their analysis and M.I.K. provided theory support. Allauthors participated in discussions of the research.

Competing financial interestsThe authors declare no competing financial interests.

Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/

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Detection of individual gas molecules adsorbed on grapheneFigure 1 Sensitivity of graphene to chemical doping.Figure 2 Constant mobility of charge carriers in graphene with increasing chemical doping.Figure 3 Single-molecule detection.ReferencesAcknowledgementsAuthor contributionsCompeting financial interests