Photo-activated oxygen sensitivity of graphene at room temperatureArtjom Berholts, Tauno Kahro, Aare Floren, Harry Alles, and Raivo Jaaniso Citation: Applied Physics Letters 105, 163111 (2014); doi: 10.1063/1.4899276 View online: http://dx.doi.org/10.1063/1.4899276 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/16?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A calibrated graphene-based chemi-sensor for sub parts-per-million NO2 detection operating at roomtemperature Appl. Phys. Lett. 104, 183502 (2014); 10.1063/1.4875557 Non-hexagonal symmetry-induced functional T graphene for the detection of carbon monoxide J. Chem. Phys. 139, 034704 (2013); 10.1063/1.4813528 Detection of organic vapors by graphene films functionalized with metallic nanoparticles J. Appl. Phys. 112, 114326 (2012); 10.1063/1.4768724 Graphene based field effect transistor for the detection of ammonia J. Appl. Phys. 112, 064304 (2012); 10.1063/1.4752272 Room temperature detection of NO2 using InSb nanowire Appl. Phys. Lett. 99, 033103 (2011); 10.1063/1.3614544

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Photo-activated oxygen sensitivity of graphene at room temperature

Artjom Berholts, Tauno Kahro, Aare Floren, Harry Alles, and Raivo Jaanisoa)

Institute of Physics, Faculty of Science and Technology, University of Tartu, Ravila 14c, Tartu 50411, Estonia

(Received 11 June 2014; accepted 10 October 2014; published online 24 October 2014)

Photo-induced changes in the electrical conductivity and the sensitivity to oxygen gas of graphene

sheets grown by chemical vapor deposition and transferred onto Al2O3 and SiO2 thin film

substrates were studied at ambient conditions. The pristine graphene sensors were initially

completely insensitive to oxygen gas at room temperature but showed significant (up to 100%)

response when illuminated with weak ultraviolet (300 nm or 365 nm) light. Oxygen response was

governed by Langmuir law and its activation was insensitive to humidity. The mechanism of

sensitization is analyzed together with other photo-induced effects—negative persistent photo-

conduction and photo-induced hysteresis of field effect transistor characteristics. While the reduc-

tion of conductivity in air is persistent effect, the oxygen sensitization and enlargement of hysteresis

take place only under the direct influence of light. It is concluded that the charge traps with differ-

ently adsorbed oxygen and water are involved in these phenomena. VC 2014 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4899276]

Graphene is a one-atom-thick carbon material, which is

considered as the main building block for the next generation

electronic and sensor applications.1 The pure two-

dimensional nature of graphene together with easy doping2

and low electrical noise of that material3 should allow the

fabrication of sensitive chemical sensors. The detection of

NO2 gas on ppq-level4 or even single adsorbed molecules3

has been successfully demonstrated in inert atmospheres.

However, in order to use the graphene-based sensors in prac-

tical applications, it is important to test them under ambient

conditions, but not many reports of such studies can be found

in literature.

Oxygen is an important gas, always present in ambient

atmosphere and playing critical role in different areas, such

as medicine, agriculture, human safety, and environmental

monitoring. The semiconductor oxygen sensors are being

widely used5 but are relatively bulky and power-consuming

because of high working temperature. Oxygen sensing with

graphene was recently demonstrated at or near room temper-

ature.6,7 However, the response was relatively small or even

absent in ambient conditions.7,8

One possibility to enhance the gas response is by ultra-

violet (UV) illumination, which has been demonstrated for

NO2 and NH3 conductometric sensors based on both carbon

nanotubes9,10 and graphene.4 The influence of UV light on

the electrical characteristics of graphene in atmospheric con-

ditions or in vacuum has been addressed in a number of stud-

ies.11–18 A common effect observed in air has been the

decrease of conductance under the influence of light,11

named as negative persistent photo-conductance.19 It has

been explained as light-induced de-doping originating from

desorbed oxygen and water molecules. It is generally

accepted that in air the graphene becomes strongly p-doped

due to oxygen adsorption, whereas the effect is stronger in

the presence of humidity.13,17,20

In the present work, we investigated the effect of UV

light on the oxygen sensitivity of CVD (chemical vapor dep-

osition) graphene sheets on SiO2 and Al2O3 thin film sub-

strates in ambient conditions. Initially, the conductivity of

sensors was almost insensitive to oxygen gas (relative

response <0.1%), but the response was activated with rela-

tively weak illumination by UV light. We also demonstrate

that the mechanism of photosensitization is different from

that causing the commonly observed negative photo-

conductance of graphene.

The graphene growth was carried out in a hot wall

quartz tube CVD reactor. Copper foils (25 lm thick; 99.5%;

Alfa Aesar) were annealed typically for 25–40 min at

950 �C–1000 �C in Ar/H2 (both 99.999%) and then exposed

to the mixture of 10% CH4 in Ar (99.999%) at the same tem-

perature range for 25–45 min. The graphene was transferred

to a target substrate wet chemically. First, a thin layer of pol-

y(methyl methacrylate) (PMMA, Alfa Aesar) was spin

coated on top of graphene films on copper foils. PMMA/gra-

phene/Cu foils were then floated on 0.1M aqueous FeCl3 or

(NH4)2S2O8 to dissolve Cu foil. The suspended films were

transferred to deionized water to remove the residual copper

etchant. After that, graphene films were transferred onto sub-

strates, Si/SiO2 with 285 nm thick thermal oxide layer or

Si/SiO2 þ 10–20 nm thick Al2O3 grown by atomic layer dep-

osition. The Ti(3 nm)/Au(60 nm) electrodes made by elec-

tron beam deposition through a shadow mask were situated

either below or on the top of the graphene layer. The dimen-

sions of slit between the electrodes were typically

0.1� 2 mm or 1� 4 mm. The scheme of a sensor with elec-

trodes deposited on the substrate is shown in Fig. 1(a). In

order to form a back gate electrode with a good contact, the

back side of Si plate was cleaned with HF and a Ti/Au layer

was evaporated onto this side before all other treatments.

The samples were characterized by optical microscopy,

scanning electron microscopy (SEM; Helios NanoLab 600),

and Raman spectroscopy (Renishaw inVia, 514.5 nm

excitation). Fig. 1(b) presents a typical SEM image with

a)Author to whom correspondence should be addressed. Electronic mail:

raivo.jaaniso@ut.ee.

0003-6951/2014/105(16)/163111/5/$30.00 VC 2014 AIP Publishing LLC105, 163111-1

APPLIED PHYSICS LETTERS 105, 163111 (2014)

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distinguished grain boundaries and some relatively small

darker areas of multiple layer graphene. Fig. 1(c) shows a

typical Raman spectrum from graphene between the electro-

des. The locations of G- and 2D-bands were generally at

1583–1590 cm�1 and 2680–2690 cm�1 and their full widths

at half maximums were 14–20 cm�1 and 30–40 cm�1, respec-

tively. 2D-band had a single Lorentzian shape. D-band, that

reveals the presence of crystalline defects, was observed

infrequently, commonly close to Au electrodes or at the edges

of graphene. Raman two-dimensional mapping (see Fig.

1(d)) showed that the area ratio between 2D- and G-bands

was commonly equal to 5–6.

The measurements of electrical characteristics and gas

sensitivity were carried out with sourcemeters (Keithley 2400

and 5450), gas mixing system based on mass flow controllers

(Brooks, model SLA5820), and a sample chamber with a vol-

ume of 7 cm3. The voltage applied between the electrodes was

10–100 mV. The gases used in our measurements, N2 and O2,

were both 99.999% pure. The gas flow through the sample

chamber was kept constant at 200 sccm and the ratio between

the flow rates of two gases was varied for changing the oxy-

gen content. For adjusting the humidity level, a parallel flow

channel with water bubbler was used for N2 gas. As a light

source, the Xe-Hg lamp (Hamamatsu) was used, in which the

infrared light was cut off by a water-filled filter, and a proper

UV wavelength (300 nm or 365 nm) was selected with a

narrow-band interference filter (Andover). The intensity of

light on the sample was 10–20 mW/cm2.

Figure 2 shows the temporal evolution of sensor current

in different gas environments with and without illumination

of the sample by UV (k¼ 365 nm) light at room temperature.

It can be seen that the conductivity decreased under the influ-

ence of optical excitation and started to increase slowly after

the illumination was stopped. The gas composition was

changed three times during the experiment—the white areas

in Fig. 2 correspond to synthetic air (a mixture of O2 and N2

in the ratio of 21:79) and the grey areas to pure nitrogen gas.

For all samples studied, the oxygen sensitivity was initially

absent (relative response< 0.1%) in dark at room tempera-

ture. In Fig. 2, no signal change can be seen within the first

FIG. 1. (a) Schematic illustration of gas sensor based on CVD graphene; (b) SEM image of graphene between the Au/Ti electrodes; (c) typical Raman spec-

trum of graphene (klaser¼ 514.5 nm); (d) Raman map of S2D/SG ratio collected from the sensor surface with a step size 0.25-mm along x- and 0.5 mm along y-

coordinate.

FIG. 2. The temporal evaluation of graphene sensor’s current in dark and

under illumination in different gas environments. The gas composition was

varied between synthetic air (white areas) and pure nitrogen (three shaded

time-windows). Two curves correspond to the measurements at different rel-

ative humidity levels: (a) RH¼ 0% and (b) RH¼ 50%. The wavelength of

light was 365 nm and its intensity was 20 mW/cm2. The substrate was

Al2O3/SiO2/Si, T¼ 27 �C. Curve (a) is shifted upwards by 5 lA for clarity.

163111-2 Berholts et al. Appl. Phys. Lett. 105, 163111 (2014)

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shaded time-window. When the light was switched on, the

conductivity decreased to about 50% level of its initial value.

At this stage (the second gray time-window), one observes

reversible >25% response to a change from air to nitrogen.

Finally, when the light was switched off, the signal started to

increase slowly and oxygen response practically vanished

again. Consequently, the removal of oxygen from the envi-

ronment resulted the sensor response only under the influ-

ence of light, whereas the effect was similar in dry (curve

(a), RH (relative humidity)¼ 0%) and humid (curve (b),

RH¼ 50%) gas. The effect was common to all samples stud-

ied and did neither depend qualitatively on substrate material

(SiO2 or Al2O3) nor on position of the electrodes (below or

on top of graphene). The response times (at changing the gas

composition from nitrogen to air) as determined by fitting

the temporal curves with exponential function were

200–300 s for both substrates studied. The recovery times (at

changing the oxygen containing gas to pure nitrogen) were

somewhat different, being 400 s for Al2O3 substrate and

750 s for SiO2 substrate. The difference in recovery times

may be explained by the influence of charge transfer between

graphene and substrate traps, which is induced by Fermi

level shift at desorption and is occurring with different time

constants in case of two substrates.

Figure 3 demonstrates the repeatability and concentra-

tion dependence of the light-activated oxygen sensitivity. In

Fig. 3(a), three different relative responses are shown, meas-

ured before (1), under (2), and after (3) illumination with UV

light (k¼ 300 nm; 10 mW/cm2). The measurements were

made at room temperature (T¼ 25 �C) by changing twice the

gas flow from dry synthetic air to nitrogen gas.

In Fig. 3(b), the series of responses to different oxygen

contents in the O2/N2 mixture are shown. In this experiment,

the sample was initially held in nitrogen gas, then the illumi-

nation was switched on, and after the signal was stabilized to

a lower level the gas was changed between nitrogen and O2/

N2 mixtures with different composition. The responses

increased linearly at low oxygen concentrations but started

to saturate at approaching 100% oxygen content. The con-

centration dependence of the relative response could be well

approximated by a Langmuir law

DI

I¼ cx

1þ bx; (1)

where x is oxygen content in the gas, c is the sensitivity at

low oxygen concentrations, and b is the affinity constant.

The fitting of the experimental results is presented in inset of

Fig. 3(b) and yielded the following values of parameters:

c¼ 0.05 1/% and b¼ 0.045 1/%. The Langmuir dependence

(1) is a natural law for homogeneous material with a clearly

defined adsorption site. Note that in recent studies of oxygen

sensitivity in TiO2/graphene composites very different con-

centration dependences were observed: either linear8 or

logarithmic.21

When analyzing the mechanism of oxygen sensitivity

and its photo-activation, one should look, in parallel, at the

other light-induced phenomena as well. The first such effect

is the decrease of the conductivity under illumination (persis-

tent negative photo-conductance19), which can be seen in

Figs. 2 and 3(b). This effect has been interpreted as the result

of oxygen/water photo-desorption. In ambient conditions,

graphene is p-doped because of these species and hence their

desorption will lead to the decrease of hole-conductivity.

Note that oxygen adsorption on regular graphene planes is

unstable at room temperature (adsorption energy Ea< 0.2 eV,

Refs. 22 and 23) but it can be adsorbed on impurities

(Ea¼ 1–4 eV, Refs. 22 and 23), at the edges,24,25 and as

oxygen-water electrochemical couple (O2þ 4Hþþ 4e� $2H2O).13 The fact that both the water and oxygen are

involved in photo-induced conductivity changes and Dirac

point shifting has been clearly demonstrated by Yang and

Murali.26 From our experiments, a similar conclusion can be

made, as the recovery of the signal after illumination

occurred only in the presence of oxygen and was faster in the

humid air (see Fig. 2). Total recovery of the signal occurred

after holding the samples during several days in air. Similar

reversible changes have been observed in the work function27

and Raman spectra28 of UV-irradiated graphene. When our

samples were held in nitrogen after switching off the illumi-

nation, no recovery of the signal occurred in dark.

The p-doping of our samples was directly proved by

measuring the dependences of drain-to-source current Ids

from the back gate voltage UG. The Ids-UG transfer charac-

teristics in ambient conditions (room temperature and oxy-

gen containing environment) were unipolar or had always a

minimum towards positive gate voltages. An example, meas-

ured on our sample with Al2O3/SiO2/Si/Ti/Au substrate in

dry synthetic air, is given in Fig. 4. Curve (a) is recorded

FIG. 3. Repeatability (a) and concentration dependence (b) of oxygen

response. The inset shows the dependence of response amplitude on oxygen

concentration and fitting curve with Eq. (1).

163111-3 Berholts et al. Appl. Phys. Lett. 105, 163111 (2014)

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before illumination and is typical for the graphene samples

held for a while in air.17 The hysteresis of this curve indi-

cates the presence of interfacial or surface charge traps,

more precisely, the hysteresis results from the exchange of

holes between graphene and trap states or some dipolar rear-

rangements during recording. The analogous curve (b),

measured under illumination, shows that the conductivity is

reduced in accordance with previous results. One may expect

that by reducing the doping level there will be less trap states

and hence smaller hysteresis in IDS-UG measurements.

However, quite surprisingly, as one can see in Fig. 4(b), the

hysteresis is not reduced but significantly enhanced.

Obviously, the effect of illumination is not only the de-

doping of the sample through desorption of oxygen and

water but in this process also a certain number of traps are

activated due to photo-induced processes.

The other possibility for large hysteresis of Ids-UG char-

acteristic can be due dipolar rearrangements; large hysteresis

is frequently associated with the presence of water, indeed.29

In the present case, however, the water molecules are rather

desorbed and not added under optical excitation. A similar

anomalous photo-induced hysteresis was recently observed

in graphene-BN heterostructure30 and explained by photo-

generation of screening charges in BN layer between gra-

phene and substrate. The effect was persistent, lasting for

many days at room temperature. In our samples, the effect

was not persistent but could be essentially observed only

under optical illumination. Hence, the screening charges pro-

duced in our structures under optical excitation can be asso-

ciated with the activation of oxygen sensitivity, which also

lasts only during illumination.

In summary, following interpretation can be given for

light-activated oxygen sensitivity and related phenomena.

Initially, before illumination, oxygen and water are adsorbed

on graphene (graphene-oxide interface), which make it heav-

ily p-doped (UGmin> 30 V). There is some hysteresis present

in graphene field effect transistor transfer characteristic, indi-

cating the charge transfer between (substrate-related) traps

and graphene. If one changes the oxygen pressure in this

state, no change in the conductivity of graphene is observed.

Consequently, no active unoccupied adsorption sites are

available for oxygen. In other words, the coverage of active

sites h¼ 1 in the initial state before illumination. If one

applies the light, three phenomena appear: (i) oxygen sensi-

tivity; (ii) large hysteresis of Ids-UG characteristic; (iii) con-

ductivity decrease. The last effect is persistent and lasts for

tens of hours after switching the light off, whereas the oxy-

gen sensitivity and light-induced hysteresis are present only

under illumination. Desorption of both O2 and H2O should

be responsible for negative photo-conductivity as both spe-

cies are needed for complete recovery. From the data in inset

of Fig. 3, one can conclude that under the illumination the

coverage factor is h� 0.5 in air. Consequently, under optical

excitation, the desorption rate from active sites is increased

so that a dynamic equilibrium is established between adsorp-

tion and desorption processes. As follows from Fig. 2, this

equilibrium is practically not affected by humidity. We made

also special experiments where the sample was annealed in dry

air at 150 �C and the oxygen responses were recorded immedi-

ately after annealing without exposing the sample to humidity.

Exactly similar behavior of oxygen sensitivity was observed—

the response was absent before and appeared under illumina-

tion. One may conclude that the active sites responsible for

light-activated oxygen sensitivity do not involve water-related

processes but only oxygen sorption is taking place. The

adsorption energy at these sites should have sufficiently high

value to guarantee complete passivation (h¼ 1) at room tem-

perature. Regarding the origin of these sites, then the oxygen

adsorption on regular graphene lattice is unlikely: ab initio cal-

culations22,23 result in the adsorption energy Ea< 0.2 eV for

molecular oxygen and the dissociation of O2 on the surface

(with following formation of epoxy groups) is not favorable at

all unless the graphene is strongly strained.31 In principle, one

may suppose that the epoxy groups are formed as a result of

UV-assisted dissociation of O2. However, we observe the ther-

mally activated p-type sensitivity to oxygen already at

100 �C,7 while the chemisorbed atomic oxygen is shown to

desorb at much higher temperature, at 260 �C.32 Therefore, the

oxygen involved in photo-activated responses is most probably

adsorbed on graphene defects or edges,24–26 the latter view-

point was supported by observation of UV-assisted etching of

graphene flakes in air.33

To conclude, we demonstrated that oxygen sensitivity of

pristine graphene, which is almost absent in ambient condi-

tions at room temperature, can be activated by relatively

weak UV light. The conductometric oxygen response obeys

Langmuir law and is larger (almost 100% at changing nitro-

gen to oxygen) than observed previously6,8,19,34 on graphene

based structures. The activation effect did not depend on

substrate (Al2O3 or SiO2), electrode position (below or on

top of graphene), or relative humidity.

We would like to thank Lauri Aarik, Tea Avarmaa, and

Aarne Kasikov for substrate preparation; Jekaterina Kozlova

for SEM imaging; and Ahti Niilisk for help with Raman

measurements. This research was carried out with the

financial support of the European Social fund (Grant MTT1).

We also acknowledge financial support from Estonian

Research Council Grant No. IUT2-24 and by the European

Regional Development Fund (Project TK117 “High-

technology Materials for Sustainable Development”).

FIG. 4. Drain current dependence from gate voltage before (a) and under

illumination (b) in dry synthetic air. Wavelength of illuminating light

365 nm, intensity 20 mW/cm2. UG scan rate 0.067 V/s.

163111-4 Berholts et al. Appl. Phys. Lett. 105, 163111 (2014)

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