I completed my final year project with Professor Georg Duesberg's group at the Universität der Bundeswehr, München. I successfully fabricated simple chemoresistive gas sensors from thin films of platinum diselenide and demonstrated their sensitivity to both nitrogen dioxide and ammonia.

1. © School of Physics, TCD, 2019
Fabrication of High Performance Gas Sensors
from Ultrathin Platinum Diselenide Films
Mark McCrystall1, Kangho Lee2, Chanyoung Yim2, Maximilian Prechtl2 and Georg S. Duesberg2
1 School of Physics, Trinity College Dublin
2 Institute of Physics, Universität der Bundeswehr München
Sensor Fabrication
Abstract
To facilitate the further development of so-called “smart” interconnected technologies there is a demand for high-performance gas sensors which can be cheaply fabricated and easily
integrated with existing technologies. Chemiresistors based on 2D layered materials have shown the most promise but difficulties in large scale synthesis has hampered their uptake. Here, a
simple scalable method is presented for the synthesis of platinum diselenide (PtSe2), a little studied group 10 transition metal dichalcogenide, under conditions which are compatible with back-
end-of-line processing. In order to demonstrate the gas sensing abilities of this material, a device is fabricated on a 15mm SiO2 substrate and its sensitivity to NH3 and NO2 is shown.
Zone 1: 230˚C Zone 2: 450˚C
Sputtered Pt
Substrate
Selenium Source
H2 15 sccm
Chemiresistors are simple electrical devices based on a sensing material which displays a
concentration dependent change in electrical conductivity upon exposure to certain
chemical species [1]. This is caused by charge transfer due to adsorption of analyte
molecules on the sensing surface by either the formation of chemical bonds
(chemisorption) or by attractive Van der Waals forces (physisorption). The resulting effect
on the conductivity depends on whether the material exhibits n-type or p-type behaviour
and whether the adsorbed gas molecules are reducing (electron-donating) or oxidising in
nature (electron-withdrawing). Thin films of PtSe2 exhibit p-type semiconducting
behaviour and so are expected to demonstrate an increase in electrical resistance upon
adsorption of reducing molecules such as NH3, and a decrease upon adsorption of
oxidising molecules such as NO2 [2]. Older chemiresistors based on metal oxide
semiconductors operate by a chemisorption mechanism which requires high operating
temperatures. In contrast, newer 2D materials operate mainly by physisorption enabling
room temperature operation, lower power consumption and faster recovery times.
NH3
e- donating
NO2
e- withdrawing
This project would not have been possible without the assistance of: Dr Kangho Lee, who assisted with the testing of
the sensor; Dr Chanyoung Yim, who provided instruction in the use of the ellipsometer, the thermal evaporator and
the Raman spectrometer; doctoral student Maximilan Prechtl, who guided me through all steps in the operation of
the selenization chamber; Professor Georg S. Duesberg, who kindly agreed to let me work alongside his research
group for the duration of this project; and the generous scholarship provided by Professor Duesberg and the DAAD.
[1] G. Neri, First fifty years of chemoresistive gas sensors, Chemosensors, 3 (2012) 1—20
[2] C. Yim et al., High performance hybrid electronic devices from layered PtSe2 films grown at low temperature,
ACS Nano, 10 (2016) 9550—9558
[3] M. O’Brien et al., Raman characterization of platinum diselenide thin films, 2D Mater., 3 (2016) 021004
[4] J. Suehiro et al., Schottky-type response of carbon nanotube NO2 gas sensor fabricated onto aluminium electrodes
by dielectrophoresis, Sensors and Actuators B, 114 (2006) 943—949
Acknowledgements & References
Results & Discussion
Raman spectroscopy confirmed the presence of PtSe2 on the TAC processed films and
comparison with a comprehensive Raman characterisation of TAC grown PtSe2 published in
2016 confirmed an initial platinum thickness of approximately 1nm [3]. The sensor was tested
in a custom-built gas sensing chamber at room temperature and with a voltage of 1V applied
across the device terminals. Initial testing revealed non-linear IV characteristics and no
sensitivity to either NH3 or NO2. However after an hour of gentle annealing at 95˚C, linear IV
behaviour was observed in nine out of eleven devices and sensitivity was shown to both gas
species. It is thought this may be due to desorption of contaminants on the PtSe2 surface.
Graphs (a) and (b) illustrate the typical gas sensing results obtained. Unexpectedly, the sensor
responded to both gas species with an increase in resistance. Similar behaviour has been
reported in p-type carbon nanotube (CNT) sensors contacted with Al, which the authors
proposed was due to Schottky barrier formation caused by metal electrode work-function
modulation due to adsorbed NO2 molecules on the metal surface. This resistance-increasing
effect dominated the sensor response, dwarfing the smaller resistance-decreasing
contribution due to molecules adsorbed on the CNT surface [4]. The sluggish recovery shown
in the NH3 sensing experiments suggests chemisorption of species at defective sites situated
at domain boundaries in addition to physisorption at the basal plane. Despite the surprising
response to NO2 the sensor displayed high sensitivity to both gases. In both cases the signal-
to-noise ratio (SNR) was plotted against the analyte concentration, enabling the limit of
detection to estimated by fitting to an exponential model as shown in graphs (c) and (d). A
SNR of 3 is considered to be the minimum threshold for a signal to be clearly distinguished. In
conclusion, a simple, scalable and low-temperature method of synthesising PtSe2 was shown
to produce films which show great promise for application as chemiresistive gas sensors.
Background & Theory
A thin film of platinum was deposited on a 15mm SiO2
substrate by sputter deposition. A shadow mask was used to
confine the sputtered platinum to a 10mm x 2mm channel in
the centre of the substrate, outlined on the image to the right.
The thickness of the platinum film was estimated to be 0.7nm
(or 7Å) by ellipsometry. The platinum channel was converted
to PtSe2 over the course of two hours in a specially designed
chamber by a thermally assisted conversion (TAC) method
depicted below. Solid selenium was heated to its melting point, producing vaporised
particles which were carried by hydrogen gas at a flow rate of 15 sccm to a second zone
where the platinum sputtered substrates were heated to 450˚C. This method is notable
for its relatively low growth temperature which compares favourably with those typically
encountered in TMD synthesis methods, and makes the process compatible with so-called
“back-end-of-line” (BEOL) processing, meaning it can be integrated with silicon microchip
technology. Finally, twelve electrodes comprised of 30nm of nickel topped with 10nm of
gold were deposited on the PtSe2 channel by thermal evaporation and the use of a second
shadow mask, producing eleven “devices” between pairs of adjacent electrodes.
(a) (b)
(c) (d)