Solid-state Photoluminescent Quantum Dots for Explosive Detection

Federica Mitri

, Andrea De Iacovo

, Serena De Santis

, Carlo Giansante

Davide Spirito

, Giovanni Sotgiu

and Lorenzo Colace

Department of Engineering, University Roma Tre, Via Vito Volterra 62, 00146, Rome, Italy

CNR Nanotec, Istituto di Nanotecnologia, Via Monteroni, Lecce 73100, Italy

IHP - Leibniz Institut für innovative Mikroelektronik, Im Technologiepark 25, 15236 Frankfurt (Oder), Germany

carlo.giansante@nanotec.cnr.it, spirito@ihp-microelectronics.com

Keywords: Colloidal Quantum Dots, Photoluminescent Probe, Vapor Explosive Detection.

Abstract: Quantum dots are an emerging class of photoluminescent nanomaterials with peculiar properties arising from

their nanometric size that allows the occurrence of strong quantum confinement effect. In recent years, these

zero-dimensional semiconductor nanoparticles have been attracting increasing attention as luminescent probe

for optical sensing applications. However, to date, almost all quantum dots- based sensors reported in

literature exploit fluorescence from solutions whereas the development of solid-state photoluminescent

quantum dots still remains a challenge. In this paper, we demonstrate the feasibility of exploiting the solid-

state photoluminescence of PbS quantum dots deposited on a silicon substrate for vapor explosive detection,

a worrying priority for homeland security and counter-terrorism applications.

1 INTRODUCTION

Explosive-based terrorism is an ongoing challenge to

governments and societies worldwide due to the

relative ease by which these weapons can be

constructed and deployed (“Trace Chem. Sens.

Explos.,” 2006). Organizations such as JIDO (US

Department of Defense Joint Improvised-Threat

Defeat Organization), AOAV (Action on Armed

Violence), and CPOST (Chicago Project on Security

& Threats) all collect detailed and updated statistics

regarding their use and devastating effects

(Hotchkiss, 2018). The alarming data that emerged,

together with the increasing government regulations

for enhanced security screening, is driving the global

explosive trace detection market growth.

Common trace detection systems rely on

spectroscopic approaches and allow for sensitive,

selective, and fast detection. Such high performance,

https://orcid.org/0000-0002-3206-1399

https://orcid.org/0000-0001-5006-5505

https://orcid.org/0000-0001-9772-2891

https://orcid.org/0000-0003-4558-5367

https://orcid.org/0000-0002-6074-957X

https://orcid.org/0000-0003-2841-9316

https://orcid.org/0000-0002-7111-3905

however, comes at the cost of expensive and

cumbersome equipment that can often be operated

only by trained personnel. Moreover, the detection

occurs through the analysis of specifically prepared

specimen, while large environments cannot be

efficiently monitored. Recently, fluorescent gas

sensors have been proposed for the realization of

nitroaromatic compounds (NAC) detectors that can

overcome such limitations (Ma et al., 2015). Among

them, sensors based on quantum dots (QD) have been

realized exploiting their peculiar optical and

electronic properties. These are semiconductor

nanoparticles suspended in the solution phase. QD

were first developed and studied as a promising

material for photodetectors since they are easy to

synthesize and their optical properties can be easily

tuned via chemical approaches (Venettacci et al.,

2019). As all nanocrystals, they are particularly

suitable for gas sensing applications thanks to their

large surface-to-volume ratio that allows outstanding

Mitri, F., De Iacovo, A., De Santis, S., Giansante, C., Spirito, D., Sotgiu, G. and Colace, L.

Solid-state Photoluminescent Quantum Dots for Explosive Detection.

DOI: 10.5220/0010870300003121

In Proceedings of the 10th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2022), pages 48-52

ISBN: 978-989-758-554-8; ISSN: 2184-4364

reactivity even at room temperature (Galstyan, 2021;

Mitri et al., 2020). Recently, we exploited the peculiar

characteristics of PbS QD to demonstrate a

chemiresistive device for NAC detection (Mitri et al.,

2021). Due to their nanometric size, QD show strong

quantum confinement effect thus exhibiting

interesting optical properties such as intense, narrow

and size-tunable luminescence. In addition, the QD’s

surface can be sensitized with a chemical approach,

allowing selective reaction with specific analyte

molecules. Several authors already demonstrated that

amine treated-QD can be effectively employed as

luminescent probes for the selective detection of

NAC, but the proposed devices cannot detect these

compounds in the vapor phase and still need to be

operated with specific lab equipment (Akhgari et al.,

2015). To date, almost all reported QD-based devices

have involved fluorescence from solutions (Xu et al.,

2020).

Herein, we demonstrate the proof-of-concept on

employing the solid-state photoluminescence (PL)

quenching of PbS QD deposited on a silicon

substrate, as a fluorescent sensing platform for direct

detection of nitrobenzene vapor (NB, as a

representative NAC). We also demonstrate that the

evaluation of the PL quenching can be easily obtained

with low-cost and low-power electronics mounted in

a compact optical chamber. The proposed device

operates in air, at room temperature, and can detect

NB with a concentration as low as 445 ppb.

2 DEVICE FABRICATION

2.1 QD Synthesis

PbS QD were synthesized in a three-neck flask

connected to a standard Schlenk line under oxygen-

and water-free conditions. PbO (450 mg), oleic acid

(9.0 g), and 1-octadacene (3.0 g) were mixed at 80°C.

The mixture was then heated at 100°C until it became

completely transparent. The temperature was raised

to 110°C and 210µL of bis(trimethyl)sulfide diluted

in 2mL of 1-octadacene were rapidly injected in the

solution. Heating was immediately stopped, and the

solution cooled down to room temperature. The

resulting QD were precipitated, purified, and

dispersed in toluene with a 0.5 mM concentration.

The size of the QD (4.7 nm) was determined through

optical absorption measurements.

2.2 QD Deposition

Thermally oxidized silicon wafers were cut to a

typical size of 10×10mm and were cleaned in a

OH:H

(1:1) solution. After being cleaned, the

substrates were rinsed with deionized water and dried

with dry air before they were used. Devices were

fabricated with a layer-by-layer spin-coating

deposition process. Specifically, a drop of the QD

solution was deposited onto the substrate and spun at

3000 rpm for 30 s. Then, a drop of ethylenediamine

(EDA) solution in acetonitrile (10% vol.) was

deposited on the substrate and left reacting for 30 s,

before spinning at 3000 rpm for 30 s. Finally, the

substrate was washed with pure ethanol. The previous

steps were repeated for 10 times. Figure 1

schematically shows the fabrication process.

Figure 1: Schematic representation of the device fabrication

process.

The resulting QD film was 200 nm thick. The

deposited film was analysed with a scanning electron

microscope (SEM). Figure 2 shows a representative

SEM micrograph. The film appears uniform in

thickness, with several cracks and voids. Such

morphology may increase the total surface of the QD

film, thus allowing for a more efficient interaction

with the NB gas. We characterized the as-deposited

film in terms of PL spectrum. The PL was measured

exciting the film with a 532 nm laser source and

analysing its emission spectrum by means of an IR

spectrometer (Horiba iHR320) equipped with an

extended InGaAs detector and a 600 lines/mm

grating. Several PL spectra were acquired varying the

temperature of the film between -10/+50°C and the

laser power between 0.2/3mW. The resulting spectra

did not show significant variations in terms of central

wavelength, while the PL intensity was proportional

to the pump power over the whole measurement

range. Figure 3 shows a typical PL spectrum of the

QD as deposited on the silicon substrate.

Solid-state Photoluminescent Quantum Dots for Explosive Detection

Figure 2: SEM micrograph of the QD film onto the silicon

substrate.

Figure 3: Solid-state QD PL spectrum.

3 SET-UP AND MEASUREMENT

CHAMBER

The device response to NB gas was characterized

with a custom measurement set-up. The device was

kept in a closed 3D-printed measurement chamber,

with in-let and out-let connectors. Ambient air was

fluxed through the container. A closed glass container

was filled with a small amount of NB and left settling

for 1 hour to allow NB gas to reach the equilibrium

vapor concentration. A couple of manual valves

allowed the ambient air to be fluxed through the NB-

filled container before entering the measurement

chamber at a fixed flowrate of 800 mL/min. NB

concentration was varied by heating the glass

container with a water bath. The 10 mL-volume

measurement chamber contained the QD-coated

silicon substrate, a germanium photodiode (PD) and

a blue LED. The device was illuminated through the

blue LED modulated by means of a 520 Hz, 0.5 Vrms

sine oscillator whose mean value could be modulated

through a dc offset. The Ge PD was placed directly

below the device in order to detect the QD’s

fluorescence intensity by generating a proportional

photocurrent then converted to voltage through a 10

kV/A transimpedance amplifier (Thorlabs,

AMP120). Figure 4(a) shows the device’s operating

scheme. The corresponding output was demodulated

by a lock-in amplifier and data were transferred to a

personal computer. As shown in Figure 4(b), the blue

LED (emission peak at λ=465 nm) emits within the

silicon substrate’s low-transmittivity band whereas

QD’s PL (NIR PL peak at λ=1400 nm) falls within

the high-transmittivity band and the Ge photodetector

spectral response. In this way, since the Ge PD is

completely covered by the silicon substrate, just the

NIR radiation reaches the PD that, in turn, generates

a photocurrent proportional to the PL intensity.

Temperature and relative humidity (RH) were

monitored during the measurements and kept

constant at 20°C, 30% RH through room-level air

conditioning.

4 DEVICE

CHARACTERIZATION

Figure 5 shows a typical signal measured during a

complete 1.9 ppm NB gas release and purge cycle.

The device showed complete baseline recovery after

NB gas was purged from the measurement chamber.

The maximum sensor response was obtained after 10

minutes since gas release. The amplitude of the

photovoltage (PV) varied by 9.7% after 10 minutes

since gas release. The phase angle showed the same

behaviour of the amplitude with a decrease during NB

exposure.

The measurements were repeated for different NB

concentrations ranging from 445 ppb to 15.9 ppm.

Figure 6 shows the corresponding real-time

normalized PV amplitude change upon these

increasing concentrations. A nonlinear behaviour is

clearly observed, as the sensor response reaches a

saturation plateau for NB concentrations higher than

5 ppm.

PHOTOPTICS 2022 - 10th International Conference on Photonics, Optics and Laser Technology

Figure 4: a) Schematic of the optical arrangement; b) QD-film PL spectrum, Si substrate’s transmission, Ge diode’s

responsivity, and LED pump emission spectrum. All curves are normalized to their maximum.

Figure 5: Amplitude and phase angle of the photovoltage

upon sensor exposure to 1.9ppm of NB.

To investigate whether the system sensitivity may

depend on PL emission intensity, a study varying the

mean intensity of LED was performed, as reported in

Figure 7. The PL quenching showed a weak

dependence on the LED pump intensity. An optimal

value of 5.14 mW/cm

was identified. Environmental

conditions (temperature and RH) were kept stable

during all the measurements; however, given the

aforementioned stability of the PL signal over a wide

temperature range, we expect that the device can be

effectively operated in environments, as well.

Figure 6: PV amplitude upon repeated exposure to

increasing NB concentrations between 445 ppb and 15.9

ppm.

Concerning the sensing mechanism, amine-

treated QD have been already employed for sensing

NAC, assuming an electronic interaction between the

negatively charged amino-groups and the electron-

poor benzenic ring of the NB molecule, leading to the

formation of Meisenheimer complexes, as shown in

Figure 8 (Tian et al., 2017). Thus, the amine to NB

charge-transfer, with the resonating negative charge

stabilized by the withdrawing nitro group (-NO

could be responsible for the significant QD PL

quenching observed in the presence of the target gas.

Solid-state Photoluminescent Quantum Dots for Explosive Detection

Figure 7: PV amplitude towards NB (1.9 ppm) varying LED

intensity.

Figure 8: Schematic of the Meisenheimer-like amine–NB

complex.

5 CONCLUSIONS

In this paper we demonstrated the feasibility of

exploiting solid-state PL PbS QD as luminescent

device for explosive vapor detection. The device

showed good sensitivity and could detect NB vapor at

room temperature in ambient air. The lowest

measured NB concentration was 445 ppb,

corresponding to a sensor response S = 4.4% after 60

seconds exposure (S = 6.8% after 10 minutes). The

integration of QD luminescent probes into an

appropriate solid support, a silicon chip, is an

important step towards further device optimization

into a portable, miniaturized, and low-cost device for

the detection of explosives in strategic and sensitive

environments.

REFERENCES

Akhgari, F., Fattahi, H., & Oskoei, Y. M. (2015). Recent

advances in nanomaterial-based sensors for detection of

trace nitroaromatic explosives. In Sensors and

Actuators, B: Chemical (Vol. 221). Elsevier B.V.

https://doi.org/10.1016/j.snb.2015.06.146

Galstyan, V. (2021). “Quantum dots: Perspectives in next-

generation chemical gas sensors” ‒ A review. Analytica

Chimica Acta, 1152, 238192. https://doi.org/10.1016/

j.aca.2020.12.067

Hotchkiss, P. J. (2018). Explosive Threats: The Challenges

they Present and Approaches to Countering Them.

Handbook of Security Science, 2016(Dathan 2017), 1–

23. https://doi.org/10.1007/978-3-319-51761-2_19-1

Ma, Y., Wang, S., & Wang, L. (2015). Nanomaterials for

luminescence detection of nitroaromatic explosives.

TrAC - Trends in Analytical Chemistry, 65, 13–21.

https://doi.org/10.1016/j.trac.2014.09.007

Mitri, F., De Iacovo, A., De Luca, M., Pecora, A., & Colace,

L. (2020). Lead sulphide colloidal quantum dots for

room temperature NO2 gas sensors. Scientific Reports,

10(1), 1–9. https://doi.org/10.1038/s41598-020-69478-

Mitri, F., De Iacovo, A., De Santis, S., Giansante, C.,

Sotgiu, G., & Colace, L. (2021). Chemiresistive Device

for the Detection of Nitroaromatic Explosives Based on

Colloidal PbS Quantum Dots. ACS Applied Electronic

Materials, 3(7), 3234–3239. https://doi.org/10.1021/

acsaelm.1c00401

Tian, X., Peng, H., Li, Y., Yang, C., Zhou, Z., & Wang, Y.

(2017). Highly sensitive and selective paper sensor

based on carbon quantum dots for visual detection of

TNT residues in groundwater. Sensors and Actuators,

B: Chemical, 243, 1002–1009. https://doi.org/10.1016/

j.snb.2016.12.079

Trace Chemical Sensing of Explosives. (2006). In Trace

Chemical Sensing of Explosives. https://doi.org/10.10

02/0470085207

Venettacci, C., Martin-Garcia, B., Prato, M., Moreels, I., &

De Iacovo, A. (2019). Increasing responsivity and air

stability of PbS colloidal quantum dot photoconductors

with iodine surface ligands. Nanotechnology, 30(40).

https://doi.org/10.1088/1361-6528/ab2f4b

Xu, A., Wang, G., Li, Y., Dong, H., Yang, S., He, P., &

Ding, G. (2020). Carbon-Based Quantum Dots with

Solid-State Photoluminescent: Mechanism,

Implementation, and Application. Small, 16(48), 1–31.

https://doi.org/10.1002/smll.202004621

PHOTOPTICS 2022 - 10th International Conference on Photonics, Optics and Laser Technology