Visual measurement of the microscopic temperature
of porous graphene based on cholesteric liquid
crystal microcapsules
Haoyan Jiang (姜浩研)
1
, Yaoyi Tang (唐姚懿)
1
, Xiaohan Zeng (曾筱涵)
1
,
Ruiwen Xiao (肖芮文)
1
, Peng Lü (吕 鹏)
1
, Lei Wang (王 磊)
1,2,
*, and Yanqing Lu (陆延青)
2
1
College of Electronic and Optical Engineering & College of Microelectronics, Nanjing University of Posts and
Telecommunications, Nanjing 210023, China
2
National Laboratory of Solid State Microstructures, Key Laboratory of Intelligent Optical Sensing and Manipulation,
and College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China
*Corresponding author: wangl@njupt.edu.cn
Received September 24, 2019; accepted November 28, 2019; posted online February 24, 2020
Measuring the microscopic temperature of graphene is challenging. We used cholesteric liquid crystal microcap-
sules (CLCMs) as temperature sensors to detect the local temperature of three-dimensional porous graphene
through quantitative visualization. Based on a CLCM (∼20 μm in size), we determined the temperature varia-
tion in a small area with an accuracy of 0.1°C. By analyzing the color changes between two CLCMs, we dem-
onstrated the temperature changes dynamically in a region with a diameter of approximately 110 μm.
Furthermore, by comparing the color evolution among the three CLCMs, we visualized the anisotropic thermal
properties in the micro-zone. This convenient and low-cost temperature measurement method is expected to
further improve graphene-based devices.
Keywords: porous graphene; cholesteric liquid crystal microcapsule; microscopic temperature; visual
measurement.
doi: 10.3788/COL202018.031201.
Since graphene was first exfoliated from graphite
[1]
, it has
been extensively used in diverse applications, including en-
ergy storage
[2]
, single-molecule gas sensors
[3]
, and photovol-
taic (PV) cells
[4]
owing to its unique and superior electrical,
thermal, mechanical, optical, and magnetic properties
[5–8]
.
Three-dimensional (3D) porous graphene is a new type of
carbon nano-material composed of two-dimensional (2D)
graphene on a macroscopic scale. It not only inherits the
excellent properties of graphene (including high electrical
conductivity, high thermal conductivity, and chemical
stability), but it also has high specific surface area, high
porosity, excell ent compressibility, and an interconnected
conductive network owing to its special 3D micro-nano
structure. This makes it attractive for applications such
as flexible electronic equipment
[9]
, thermal engineering
[10]
,
and catalysis loading
[11]
. With the miniaturization and
integration of devices, it is important to investigate the
microscopic thermal properties to improve their perfor-
mance. 3D porous graphene can serve as an excellent
material for heat transfer in energy storage and opto-
electronic devices. However, accurately measuring the
microscopic temperature of graphene is challenging.
Conventional temperature measurement methods, such
as thermocouples, thermistors, optical fiber based temper-
ature sensors, and infrared thermometers
[12–14]
, cannot
accurately distinguish the microscopic temperature
distribution with a high spatial resolution and high
temperature sensitivity. Different approaches have been
proposed to develop ultra-small thermal sensors for
microscopic temperature measurement. These approaches
include tempe rature sensors based on carbon nanotubes
(CNTs)
[15]
; however, the resistance thermometer may
not work if the chosen CNTs are semi- or non-conductive
ones. ElShimy et al. fabricated a nano sensor through fo-
cused ion beam chemical vapor deposition (FIB-CVD) of
tungsten over atomic force microscope (AFM) cantilevers
to detect heat
[16]
. Zhong et al. measured the magnetization
of magnetic nanoparticles and obtained local temperature
information through a certain model
[17]
.Ohet al. devel-
oped an optical approach to measure the temperature dis-
tribution by exploiting the temperature dependency of the
water refractive index (RI)
[18]
. However, it cannot be
used for the microscopic temperature measurement of gra-
phene. An in-situ near-infrared (NIR) charge coupled
device (CCD) imaging system has been developed for
measuring the temperature distribution of graphene dur-
ing heating
[19]
. Although these methods provide a high spa-
tial resolution, they are expensive, complicated, and not
intuitive enough, making them unsuitable for practical
applications, particularly for 3D porous graphene.
A cholesteric liquid crystal (CLC) is a type of liquid
crystal that is sensitive to temperature. As such, the selec-
tive reflection wavelength of a CLC varies with its helix
pitch due to small temperature variations. The color
change can be directly observed in the visible band, mak-
ing the CLC an ideal temperature sensor
[20]
. Although it is
not stable, the encapsulation technique, which is a prom-
ising protective method, can be applied to overcome
COL 18(3), 031201(2020) CHINESE OPTICS LETTERS March 2020
1671-7694/2020/031201(4) 031201-1 © 2020 Chinese Optics Letters