Supplementary information for

Ultrahigh humidity sensitivity of graphene oxide

Hengchang Bi1, Kuibo Yin1, Xiao Xie1, Jing Ji1, Shu Wan1, Litao Sun1★, Mauricio

Terrones2,3★, and Mildred S. Dresselhaus4

1SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of Ministry of Education,

Southeast University, Nanjing 210096, P. R. China

2Department of Physics, Department of Materials Science, and Engineering and

Materials Research Institute, Pennsylvania State University, University Park, PA

16802

3Research Center for Exotic Nanocarbons, Shishu University, Nagano 380-8553,

Japan

4Department of Physics and Department of Electrical Engineering and Computer

Science, MIT, Cambridge, MA02139, USA

★To whom correspondence should be addressed. E-mail: slt@seu.edu.cn;

mut11@psu.edu

X-ray diffraction (XRD) patterns of the prepared G-O in our experiment are shown

in Fig. S1a. Compared with graphite (3.35 Å, 2θ=26.3°), the interlayer spacing

increased to 8.6 Å (2θ=10.07°) in pristine G-O. According to Bragg’s law, this value

indicates the existence of abundant oxygen-containing functional groups during

oxidation. Larger interlayer spacing facilitated water adsorption and desorption, thus

enhancing the response and shortening the recovery times. For the infrared (IR)

spectrum of G-O (Fig. S1b), a strong and broad absorption at 3440 cm-1 attributed to

O-H stretching vibration is seen. The C=O stretching of the COOH groups situated at

the edges of G-O sheets is observed at 1725 cm-1. The absorption attributed to O-H

bending vibrations, epoxide groups, and skeletal ring vibrations are observed at

approximately 1625 cm-1. The results, as indicated by the IR spectra, are consistent

with those of the XRD patterns (Fig. S1(a)). In addition, a number of these functional

groups are hydrophilic, including carboxyl groups, hydroxyl groups, and so on1,

which endow G-O with great capacity for water adsorption and consequently

affecting the dielectric properties of G-O.

Figure S1│ XRD patterns and IR spectrum of G-O. a: XRD characterization.

Interlayer spacing increases to 8.6 Å (2θ=10.07°) for G-O compared with graphite

(3.335 Å), according to Bragg’s law. b: IR spectrum. A large number of oxygen-

containing functional groups exist in the plane of G-O, as indicated by peaks in the

valleys in Fig. S1(b).

The first step was aimed at producing microscale interdigitated electrodes. A

schematic view of the fabrication process is shown in Fig. S2. First, the shallow

grooves with precise geometry on the silicon wafer were fabricated via the dry

etching process, that is, reactive ion etching (RIE), as shown in Fig. S2a.

Subsequently, Au was sputtered on the silicon substrate and the Pyrex7740 glass

wafer (Figs. S2b and S2c), after which the Pyrex7740 glass wafer was flip-chipped

onto the silicon substrate via the Au–Au thermal bonding process with a processing

temperature of approximately 400 °C without an electric field, as shown in Fig. S2d.

The silicon wafer was thinned to approximately 40 μm through the chemical

mechanical planarization (CMP) process (Fig. S2e). Finally, the bulk-Si

microstructures were released through the inductively coupled plasma (ICP) RIE

process (Fig. S2f). The details of the interdigitated electrodes are clearly seen in Fig.

S2g. Notably, the interdigitated electrodes were suspended in an untethered manner

(Fig. S2h).

Figure S2│Fabrication procedure for microscale interdigitated electrodes. a:

Silicon wafer with shallow grooves. b: Patterned Au films deposited on a silicon

wafer. c: Patterned Au films deposited on glass. d: Silicon wafer and glass stuck

together through Au–Au bonding. e: Reduction of the thickness of Si via the chemical

mechanical polishing (CMP) process. f: Microstructures with the expected

interdigitated electrodes released through the inductively coupled plasma (ICP)

reactive ion etching (RIE) process. g: The top view of the interdigitated electrodes. h:

The SEM images of the as-prepared interdigitated electrodes.

Figure S3│SEM images of interdigitated electrodes before and after being

covered with graphene oxide films. a: Image of electrodes without being modified

by graphene oxide films. b: Image of electrodes being covered with graphene oxide

films, even at a corner. c: Top-view image of electrodes covered with graphene oxide

films. In a word, the electrodes were totally covered by thin graphene oxide films.

We have obtained the X-ray photoelectron spectroscopy (XPS) data of G-O, and

found that the carbon-to-oxygen atomic ratio (C/O ratio) is about 2.4 and the atomic

concentration of O1s is 29.33%.

Figure S4│X-ray photoelectron spectroscopy (XPS) of G-O. The atomic

concentration of C1s and O1s is 70.67%, 29.33%, respectively.

Figure S5│Capacitance versus RH at 100 Hz, 1 kHz, and 10 kHz.

During this measurement, we increased the RH in the chamber from 15% to 95% and

then returned the RH back to 15%. With a measurement frequency of 100 Hz, it was

found that the hysteresis was close to 5%.

Figure S6│Hysteresis of the humidity sensor. The capacitance was measured by a

LCR meter, 1 V applied bias , and 25 °C.

References

1. Compto, O. C. & Nguyen, S. T. Graphene oxide, highly reduced graphene oxide,

and graphene : versatile building blocks for carbon-based materials. Small 6, 711-

723 (2010).