Lead Sulphide Colloidal Quantum Dots for Sensing Applications

A. De Iacovo, C. Venettacci, S. A. Bruno and L. Colace

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

Keywords: Pbs Colloidal Quantum Dots, Fire Detector, Gas Detector.

Abstract: Colloidal Quantum Dots (CQD) have been widely studied for their peculiar optical characteristics such as

enhanced optical absorption and tunable absorption spectrum. Many different photodetectors have been

proposed but overall performance is still poor from the point of view of the bandwidth and noise performance.

Here we propose the employment of a PbS QD photoconductor as an ultra-high sensitivity fire detector,

exploiting the outstanding device responsivity at low optical powers. Moreover, we demonstrate the

outstanding flexibility of CQD based devices, employing our detectors also as simple pollution gas sensors

for NO

detection.

1 INTRODUCTION

Colloidal Quantum Dots (CQD) are semiconductor

nanoparticles directly synthesized and dispersed in

solution. The synthesis process is straightforward and

does not require high vacuum or high temperature (Pu

et al. 2018). Moreover, the final material

characteristics can be easily tailored through the

modification of simple parameters such as reaction

time and temperature. Being nano-sized, the quantum

dots show peculiar optical, electronic and chemical

characteristics such as enhanced optical absorption

and resonant absorption spectra (Moreels et al. 2009),

ease of doping (even after material synthesis and

directly on the deposition substrate) (Kagan et al.

2016) and outstanding reactivity with several

chemical species thanks to the high surface-to-

volume ratio. Being dependent on the nanoparticle

size, all these characteristics can be easily tailored

during the QD synthesis, leading to a variety of

similar colloidal materials with a vast range of

physical properties.

Thanks to their characteristics, colloidal quantum

dots have been employed for the realization of several

different kind of devices, such as photodetectors (De

Iacovo et al. 2016), solar cells (Sargent 2012), light

emitting diodes (Caruge et al. 2008) and

chemoresistors (Liu et al. 2014). In general, all these

devices exploit the ease of formation of QD films and

either their outstanding optical properties or their very

high surface-to-volume ratio and chemical reactivity.

Among several different materials in the class of

CQDs, lead sulphide (PbS) has been widely

employed for the fabrication of near-infrared

photodetectors; it’s absorption edge can easily be

tuned, by quantum-confinement effect, from 900nm

to 1.8μm (Moreels et al. 2009) and its synthesis route

is well established, producing colloids that are stable

over a long period of time. The as-synthesized colloid

is usually stabilized with an organic, long-chained

capping agent such as oleic acid. Such colloids can be

readily deposited on a variety of substrates to produce

QD films; in this case, however, the nanoparticles in

the solid film are arranged with a mean distance twice

as long as the molecular chain of the capping agent.

Such a distance is, typically, too long for the film to

be conductive because charges cannot easily tunnel

from one dot to another. To enhance the film

conductivity, a ligand exchange procedure is usually

necessary. The long-chained ligands are stripped

away from the QD surface (either in solution or

directly from the deposited film) and substituted with

shorter ones, thus enabling charge transfer between

neighbouring nanocrystals.

This approach can be carried out on a variety of

substrates, comprising silicon and SiO

. PbS QDs

have been employed for the realization of

photodetectors and transistors directly on silicon

substrates, thus enabling their integration with silicon

electronics in a more-than-Moore framework (Balazs

et al. 2014). Nevertheless, the resulting devices still

show poor performance in terms of noise and

bandwidth and the technology is not yet comparable

De Iacovo, A., Venettacci, C., Bruno, S. and Colace, L.

Lead Sulphide Colloidal Quantum Dots for Sensing Applications.

DOI: 10.5220/0007444002350240

In Proceedings of the 7th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2019), pages 235-240

ISBN: 978-989-758-364-3

235

to other well-established more-than-Moore

approaches such as Ge-on-Si (Sorianello et al. 2010)

(Sorianello et al., 2015). Poor device performance can

usually be attributed to very low electron mobility

and high noise which, in turn, limit the device

bandwidth and detectivity (De Iacovo at al. 2017).

For these reasons, CQD based devices are more

suitable for sensing applications, where high

sensitivity and low bandwidth are required.

In this paper we show our recent work with PbS

QD devices for sensing applications both in the

optoelectronic and gas sensing field. In particular, we

propose a high-gain, visible-blind, photoconducting

photodetector for indoor flame detection and a

chemoresistive gas sensor for NO2.

2 DEVICE FABRICATION

The devices have been fabricated employing a

commercial 10mg/mL PbS QD solution in toluene

with mean particle diameter of 5nm (Sigma-Aldrich).

This particular type of nanoparticle has a first

excitonic absorption peak located near 1360nm. The

nanoparticles are capped with oleic acid and the first

fabrication step consists in the removal of the long-

chained ligand by centrifugation in excess methanol.

The precipitated nanocrystals are then washed with

methanol and any residual solvent is evaporated in a

vacuum desiccator. The QDs are redispersed in

octane with a 0.8mg/mL final concentration. The new

QD solution is drop casted on a SiO

substrate with

pre-patterned gold interdigitated contacts and the

devices are kept in a desiccator until full solvent

evaporation. Butylamine is then drop-casted onto the

QD film. In this step the amine ligates to the

nanocrystal surface and the QDs in the film are

rearranged with a short mean distance (0.6nm)

corresponding to the length of the organic molecule.

QD and butylamine drop-casting are repeated 10

times to create a nanocrystal film with e mean

thickness of 1μm. Eventually, the devices are rinsed

with methanol for 2 hours to remove all the

butylamine and improve QD packing (Konstantatos

et al., 2006). For the fabrication of the visible-blind

photodetectors, the devices are enclosed in a

packaging with a silicon optical window that acts as

a filter for visible wavelengths. Conversely, devices

meant as gas sensors are readily usable after

fabrication.

3 VISIBLE-BLIND

PHOTODETECTORS AND

FLAME SENSORS

The photodetectors where initially characterized in

terms of current-voltage characteristics to verify the

device resistivity and the ohmicity of the Au-QD

contact. Fig. 1 shows the resistance of three different

devices with interdigitated finger spacings ranging

from 5 to 20μm. As expected, the resistance increases

with the finger spacing. The devices show a linear I-

V characteristic (Fig. 1, inset) confirming the

ohmicity of the Au-QD contact.

Figure 1: Device resistivity vs. interdigitated finger

spacing. Inset: voltage-current characteristics of a 5μm

device.

A semiconductor laser at 1300nm has been employed

for the characterization of the device’s

photoresponse. Fig. 2 shows the measured

responsivity and its dependence on the applied bias.

As expected for a photoconductor, responsivity is

proportional to the applied bias and at 1V it is higher

than 1A/W. Thus, the photodetector shows a

photoconductive gain. This phenomenon has been

previously observed in similar devices and should be

attributed to the long electron lifetime due to the

presence of deep electron traps with a reduced cross-

section for holes (Konstantatos et al. 2007). The

presence of such a trapping mechanism usually

induces a nonlinear response in the photodetector. At

low incident optical power, in fact, all the

photogenerated electrons can be trapped and the

photoconductive gain is maximum; conversely, when

the photogenerated carriers outnumber the available

5 10 15 20 25 30 35

Resistance [



Finger spacing [m]

-1.5 10

-6

-1 10

-6

-5 10

-7

5 10

-7

1 10

-6

1.5 10

-6

-1.5 -1 -0.5 0 0.5 1 1.5

Current [A]

Applied bias [V]

PHOTOPTICS 2019 - 7th International Conference on Photonics, Optics and Laser Technology

236

trap states, some electron can be swept throughout the

device and can be collected at the gold contacts in a

time much shorter than the mean electron lifetime. In

this case the overall photoconductive gain is reduced.

In order to verify this behavior, we characterized the

device varying the incident optical power. Fig. 3

shows the obtained R vs. P characteristics. As

Figure 2: Responsivity vs. voltage characteristics of a 5μm

photodetector.

expected, the responsivity has a strong dependence on

the optical power and reaches a plateau around 5nW.

It should be observed that, given the low operating

voltage, our devices show a responsivity performance

comparable with state-of-the-art PbS QD

photoconductors (Saran et al. 2016).

The high responsivity at low incident optical

power can be exploited for the realization of detectors

for fire and flame sensing applications. It is well

known that a hot body emits an optical radiation

whose spectrum is described by Planck’s law as

defined in (1), where B is the spectral irradiance at the

optical frequency υ and temperature T, h is the Planck

constant, K is the Boltzmann constant and c is the

speed of light.

  

















 

(1)

A flame, burning between 900 and 1100°C

(Babrauskas 1980), emits a radiation with a wide

spectrum from the visible wavelengths to the infrared,

peaked at around 1800nm. Being highly sensitive in

the NIR region of the spectrum, the PbS CQD

photoconductor can be easily employed for flame

detection but, in order to avoid false detection due to

ambient illumination, a visible light filter must be

included. As previously mentioned, we encapsulated

the photodetector in a packaging provided with a

silicon window which completely absorbs radiation

in the visible range. Fig. 4 shows both the spectral

responses of the unpackaged (red curve) and

packaged (blue curve) photodetector. The silicon

filter absorbs all the light below 900nm. Fig. 4 also

shows the presence of the excitonic absorption peak

of the PbS QDs at 1360nm.

Figure 3: Responsivity vs. incident optical power.

The removal of any radiation with λ<900nm implies

that the device can be effectively employed for flame

detection in an indoor environment. Light sources used

in domestic environment, in fact, don’t emit any

infrared radiation that could interfere with the flame

detection. Conversely, sunlight could prevent the

detector’s operation. We tested our device in a closed

room with standard office illumination (neon lamps,

250 lm/m2) lighting a wax candle in front of the

photodetector and varying the distance between the

candle and the detector surface. Fig. 5 shows the

percent variation of the measured current with respect

to the dark current at 1V bias; as expected,

photocurrent follows an inverse square law with the

flame distance.

We also measured the mean dark current and its

standard deviation over 10 minutes, obtaining σ =

0.25%. We defined a flame detection threshold that is

20 times larger than σ (5%). This threshold and its

intercept with the detection curve are represented

with dashed lines in Fig. 5. The detection threshold is

reached when the candle is lit 17.9m from the

detector’s surface.

0.5

1.5

2.5

-1.5 -1 -0.5 0 0.5 1 1.5

Responsivity [A/W]

Applied bias [V]

opt

= 1W @ 1300nm

1 10 100 1000

Responsivity [A/W]

Optical power [nW]

Applied bias = 1V

 = 1300nm

Lead Sulphide Colloidal Quantum Dots for Sensing Applications

237

Figure 4: Spectral response of the PbS CQD photodetector

with (blue curve) and without (red curve) the silicon filter.

Figure 5: Detector current variation vs. candle flame

distance.

Given the device resistance and the applied

voltage, we also evaluated the mean power

dissipation of the flame detector as P

diss

= V

/R≈

1.2μW.

Eventually, we verified that the detector response

was not modified with respect to environment

illumination. We tested our device from complete

dark to strong indoor illumination (neon and LED

lamps, 1000-2000 lm/m

) and could not observe any

difference in the detector behavior.

Our results demonstrate that a PbS QD

photodetector can be effectively employed as a high-

sensitivity flame detector for indoor safety systems,

providing very high detection distance and low power

consumption.

4 POLLUTION GAS SENSOR

Figure 6: PbS QD sensor resistance variation in response to

50ppmmol NO

QD surface chemistry plays a key role in the

determination of the electronic characteristics of

CQD electron devices (Brown et al., 2014); oxidation

of the QD film, in particular, has shown dramatic

effects on the performance of PbS photodetectors (De

Iacovo et al., 2016). One of the main class of polluting

gases is represented by nitrous oxides (NO

) and we

tested our devices as gas sensors for NO

detection.

The devices were wire bonded to a custom chip

carrier and inserted into an enclosed test chamber

where we fluxed pure nitrogen and NO

, varying the

proportional flux of the pollutant gas. Fig. 6 shows

the typical resistance variation of a PbS QD resistor

(finger spacing = 20μm) in response to 50ppmmol of

. The sensor shows a slow response to the gas,

reaching a resistance plateau after 10 minutes of

fluxing. Also, gas desorption is very slow and

complete recovery happens only after 1.5 hours.

We evaluated the sensor response to different gas

concentrations. Fig. 7 shows the percent resistance

variation with respect to the ppm of NO

fluxed into

the measurement chamber and the corresponding

linear fit. We evaluated a resistance variation of

1.5%/ppm corresponding to 3MΩ/pmm.

Our results are promising for future development

of PbS CQD based sensors for pollutant gas detection,

nevertheless, the slow response are recovery time are

still an issue. A possible approach to enhance device

response and recovery time should consist in a

reduction of the thickness of the QD film.

2000

4000

6000

8000

1 10

1.2 10

880 960 1040 1120 1200 1280 1360 1440

Absorption (w/o Si filter)

Absorption (w/ Si filter)

Photoresponse [a.u.]

Wavelength [nm]

100

150

200

250

300

0 5 10 15 20 25 30

flame

dark

[%]

Flame distance [m]

100

0 1000 2000 3000 4000 5000

R [%]

Time [s]

Gas in

Gas out

50ppmmol NO

PHOTOPTICS 2019 - 7th International Conference on Photonics, Optics and Laser Technology

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Figure 7: Device resistance variation for different NO

concentrations. Experimental data (points) and linear fit

(line).

We determined the sensor noise by continuously

measuring its resistance with no gas flow for 10

minutes and determining the standard deviation of the

acquired data. We employed this data to extrapolate

the theoretical detection limit of our sensor obtaining

a minimum detectable NO2 concentration of 4.9ppb.

Even if our detector shows lower performance if

compared to other PbS QD based NO

sensors (Song

et al., 2018), it should be noted that we propose a

device that is already integrated on a Si/SiO

substrate

and that, consequently, could be easily coupled with

readout electronics to produce a standalone NO2 gas

sensor with a more-than-Moore approach. Moreover,

device parameters such as QD film thickness and

metal finger spacing could be optimized to enhance

the device response. This analysis is, however, out of

the scope of this paper and will be discussed in future

publications.

5 CONCLUSIONS

In this paper we showed our results with optical and

chemical sensors based on PbS colloidal quantum

dots. We showed how the same CQD device, with

only slight modifications in the fabrication process,

can have manifold applications for different sensing

purposes. We proposed a novel fire detector with

ultra-high sensitivity for indoor applications and a

pollution gas sensor integrated on a Si/SiO

substrate,

defining a strategy for future sensor integration with

silicon electronics.

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