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
1
SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of Ministry of Education,
Southeast University, Nanjing 210096, P. R. China
2
Department of Physics, Department of Materials Science, and Engineering and
Materials Research Institute, Pennsylvania State University, University Park, PA
16802
3
Research Center for Exotic Nanocarbons, Shishu University, Nagano 380-8553,
Japan
4
Department 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).