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Room Temperature MIR Detection through Localized Surface Vibrational States of SnTe Nanocrystals Matthew E. Cryer, and Jonathan E. Halpert ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b00448 • Publication Date (Web): 26 Sep 2018 Downloaded from http://pubs.acs.org on September 27, 2018

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Room Temperature MIR Detection through Localized Surface Vibrational States of SnTe Nanocrystals

Matthew E. Cryer1 and Jonathan E. Halpert1,§,* 1

MacDiarmid Institute for Advanced Materials and Nanotechnology, School of Chemical and

Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand. M.E. Cryer ORCID ID: 0000-0002-4876-4195 J.E. Halpert ORCID ID: 0000-0002-9499-2658 E-mail: [email protected]; [email protected] *corresponding author(s): J.E.H. § see Author Information for present address

Keywords: mid-infrared, nanocrystals, detection, surface states, device 1 ACS Paragon Plus Environment

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Abstract Quantum dots (QDs) are now well established as promising materials for room temperature midinfrared (MIR) detection beyond 3 µm. Here, we have replaced commonly reported mercury based quantum dots with less toxic SnTe and PbSnTe. Inverse MIR detection at room temperature is demonstrated with planar, solution and air-processed PbSnTe and SnTe QD devices. The detection mechanism is shown to be mediated by an interaction between MIR radiation and the vibrational stretches of adsorbed hydroxyl species. Devices are shown to possess mA/W responsivity via a reduction in conductance due to MIR irradiation and, unlike classic MIR photoconductors, are unaffected by visible wavelengths. As such these devices offer the possibility of MIR thermal imaging that has an intrinsic solution to the blinding caused by higher energy light sources.

Main Text Colloidal quantum dot (CQD) photodetectors offer the promises of cheap, simple fabrication with bandgap tunability and room temperature operation.1–5 The most recent advances in the use of HgTe CQDs for MIR detection have seen increased responsivities using relatively complex photo-FET structures6 and nano-antenna arrays.7 Real-time imaging has been demonstrated with a cooled, planar device,8 and HgSe has been used to demonstrate plasmon structure enhanced, narrowband MIR detection.9,10 For these devices, visible light must still be filtered out, since both HgTe and HgSe are photosensitive to any photon with energy greater than the band-gap. Additionally, mercury is a toxic material that poses risks to health and the environment and is unlikely to be suitable for mass market, consumer applications.11

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SnTe is relatively non-toxic, has a bulk bandgap of 0.18 eV12 and can be made into a zero bandgap material in the bulk, due to a bandgap crossing when alloyed with lead.13 Additionally, the band structure of SnTe QDs is similar to HgTe QDs which has tuneable bandgaps in the MIR and LWIR,14 and would therefore appear to be an ideal candidate for a mercury free device. However, even with the numerous published reports of mercury-free, MIR absorbing CQDs13,15– 17

there are no reports of successful MIR photodetectors using these materials.17–20 Similarly,

quantum confinement21 and band-edge tuning in Hg-based materials remains the only aspect of QD physics that has been effectively utilised in MIR photodetectors.22 Even though there have been many reports of QDs demonstrating other physical effects in the MIR, such as localised surface plasmon resonance (LSPR)23–26 and surface interactions with adsorbed species,27–29 none of these have yet yielded a functioning device. The surface interface between neighbouring QDs is known to have a large influence on the conductance of QD thin films.30,31 Yet the surface of MIR CQDs has always been treated as an area to passivate rather than utilise for detection. MIR absorption peaks have been shown in various reports of SnTe17–20 and PbSnTe13 QDs and have been variously ascribed to bandgap absorption,13,17 LSPR absorption19 and ligand absorption.20 Here we demonstrate that for films made from 3 sizes of SnTe QDs, as well as in PbSnTe QDs,13 there is an absorption peak that can be used to detect MIR photons due to changes in the resistance of the film. We are able to show that this is not a band edge effect because only IR light that is resonant with the surface absorption feature can be detected. Not only is this the first reported instance of MIR detection using SnTe CQDs but also represents the first device to use MIR QD surface species to sense incident MIR radiation though a simple twoterminal measurement of current. This effect cannot be seen in the bulk semiconductor or nanoparticles of tin oxide. Although PbSnTe QDs are more toxic than pure SnTe QDs they are


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included here as they represent one of the few published MIR absorbing QDs and, due to their differing band edge from SnTe, allow a greater understanding of the underlying mechanism. Results and Discussion PbSnTe, PbTe and SnTe QDs were synthesized via a synthesis adapted from Euliss et al.32 and formed dark black suspensions that were stable in air for months. The samples were characterised by XRD and the cubic halite crystalline structure was observed (Figure S1). The attenuated total reflectance (ATR) absorption spectra of drop cast films is shown in Figure 1A. The absorption spectra here are similar to those reported previously for these materials.13,17–20 The largest SnTe QDs at 13.6 nm did not make reliable devices but are included to show the band edge progression with size. In addition to the band edge, there is also a broad absorption peak around 3200 cm-1 (3125 nm, from 2700 nm to ~5000 nm) that does not vary in its position with reaction conditions or QD size. Films of the colloids were successfully ligand exchanged with the infrared inactive material As2S3, to increase the conductivity of the thin film (Figure S2 and Figure S3). Both As2S3 and SnO2 are wide bandgap semiconductors and do not absorb in the IR.33–35 The near-infrared (NIR)-MIR absorbance of the films (normalized at 12000 cm-1) is shown in Figure 1B. In Figure 1B the absorbance feature at 3200 cm-1 is present in all samples and the band edge of the QD films is also clearly visible, importantly the band edge of the PbSnTe film drops to zero before 4000 cm-1 and it can therefore be seen that the feature at 3200 cm-1 is separate to the band edge absorption of the nanocrystal. Additionally, the pseudo-excitonic peak in the SnTe QD films is in fact a juxtaposition of the 3200 cm-1 feature and the monotonically decreasing band edge rather, rather than representing the band edge excitonic states of a highly confined QD. Considering the size dispersions of these colloids (Figure S4 – S8) it would not be expected than an excitonic 4 ACS Paragon Plus Environment

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peak is visible. Certainly it would not be at exactly the same position (red dashed line in Figure 1A and Figure 1B) for all the sizes of QD. In Figure 1A, it can be seen that the SnTe band edge absorption is relatively a lot larger than the feature absorption. In TEM images (Figure S4 – S8) SnTe and PbSnTe lattice fringes are clearly evident and show lattice spacing that is attributed to the halite structure observed by XRD. It is expected that the Sn surfaces would quickly oxidise.36 However, there are no tin oxide peaks visible in the XRD data (Figure S1) so oxidation is presumed to be limited to a small region at the surface, rather than individual SnO2 nanoparticles (NPs) or other large crystalline volumes. TEM images also show a shell region with discontinuous lattice fringes (Figure S9). STEM mapping (shown in Figure S10 and Figure S11) confirms that all of the intended precursors are localised to the QDs, indicating that TEM EDS measurements are accurately measuring QD composition. SEM EDS (Table S1), TEM EDS and ICP-MS (Table S2) show a consistent stoichiometric Te deficiency that indicates a Sn rich surface with significant oxidation. For electrical measurements the QDs (20 mg/ml) were spin-coated onto gold interdigitated electrode (IDE) structures on Si/SiO2 (Figure 1C), producing an even film (Figure 1D). The ligand exchange increased the conduction of all the SnTe QD films by at least 3 orders of magnitude giving device signals in the µA range (Figure S2). Control devices were made with no QDs, SnO2 NPs and HgTe QDs and these were investigated with and without ligand exchange. None of the control devices showed behaviour similar to the Sn-based QD devices. The HgTe devices were photosensitive to the MIR, but these showed normal photoconductive behaviour. Figure 2A shows a typical set of IV curves for the SnTe and PbSnTe QD devices, with and without MIR radiation, operated at room temperature in air. None of the Sn-based devices displayed a photoresponse to visible light. However, when exposed to a broadband IR source 5 ACS Paragon Plus Environment

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generated by a blackbody (BB) source the films demonstrated a perfectly time-correlated decrease in conductivity (Figure 2C - inset). The magnitude of the conductivity decrease was proportional to the intensity of the incident radiation between 2.8 and 5 µm. When the IR was removed the conductivity returned to the original level. The incident flux is controlled via longpass filters that spectrally cut the incident light and as a result also modulate the total incident power. This confirms that there is both a proportional electrical response to the incident light and more importantly, that it is observed when only MIR photons are present, as shown in Figure 2C. From a performance perspective, an inverse photoresponse has a clearly limited specific detectivity, due to its inherently high noise level. However, both the mechanism and the zero current response to visible light offer valuable insight into QD film physics and perhaps suggest a way to produce novel light and even analyte sensors using surface species. Figure 2D shows the responsivity versus bias for each QD sample to incident BB spectra with a maximum photon energy of 410 meV, with a 10 s settle time. The highest responsivity was found for the 11.4 nm SnTe QD device. The initial device value of 2 mA/W at wavelengths above 3 µm, at room temperature, with 5 V bias is only one order of magnitude less than the best reported equivalent, fully air processed HgTe device.3 Figure 2C inset shows the current response to the MIR source being chopped at 4 Hz (10 Hz response in Figure S14). The response time of these devices is slow compared to the best HgTe sensors,2,37 but better than reported HgSe devices.38 Any discussion of the mechanism in these devices must clearly address two issues; the MIR absorption feature and the inverse photoresponse of the devices. As is shown, the photoresponse only occurs for IR light that overlaps the absorption feature around 3200 cm-1, and not for light sources at higher energies (λ 300 °C) here would sinter the film and aggregate the QDs. However, it has been found that the weakly bound associated hydroxyls can be temporarily replaced with OD (deuterium) groups if exposed to a high concentration of D2O at room temperature, as shown in Figure 3C. The rooted Sn–OH stretch is unaffected, but when there is a measurable O-D stretch (right hand line) there is also a reduction in the absorbance region for the associated hydroxyl (left hand line). It has been reported that the rooted OH group remains on SnO2 surfaces even after outgassing at 773 K and major OD replacement has only been achieved after 48 h of D2O vapour at 633 K.44,51 Additionally, the only surface groups that would be available for this exchange are those associated groups that are not morphologically encapsulated in the film and hence, for both of these reasons, full exchange would not be expected. The total intensity of the absorbance loss at the O-H position matches closely to the increased absorbance at the O-D position. After several minutes in ambient atmosphere the OD group disappears and the feature regains its original shape. The presence of the increased donor electron density at the surface increases the conductivity of the film, and, as described above, the now ionized, rooted HOlat group has a molecular 9 ACS Paragon Plus Environment

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vibrational stretch in the MIR.29,43,48,49 When this is excited by incident radiation there is a vibronic coupling between the vibrational state of the Sn-OH stretch and the bulk electrical states that creates a new localised state.28,52–54 Ostensibly, this state can trap a free electron, thus reducing the effective carrier density and decreasing conductivity of the film. This model is shown in Figure 4A, and although not previously observed in tin chalcogenides specifically, explains the inverse photoresponse, the MIR absorption, the resonant electrical coupling and the behaviour of the device in wet air, dry nitrogen, and vacuum atmospheres. The presence of adsorbed species from hydroxyl groups on the (oxidized) surface explains the change in conduction by a change in the carrier density of the nanocrystal.46–48 This type of surface doping has been well established for a variety of oxide surfaces commonly used for reversible gas sensing.27,49,55 The mechanism of carrier doping involves the change in available carriers due to the specific species that is bound at the surface.27,55–57 This model is illustrated by Figure 5A and Figure 5B which shows the absorption of treated SnTe and PbSnTe QD films in air and in vacuum, respectively. In both films the absorbance is identical apart from the OH caused feature around 3200 cm-1. This suggests that a proportion of the absorbing species are reversibly removed by placing the film in a vacuum (or more slowly, in dry nitrogen), which indicates that surface species are playing a dominant role in the conductivity. This, coupled with the fact that the associated hydroxyl group can be replaced with an OD group strongly suggests that surface groups are responsible for conduction and that these surface groups are water based. Since there is no conventional photoconductive behaviour observed for SnTe, it is reasonable to conclude that photo-induced carriers in the SnTe band structure are not contributing strongly to conduction, probably due to a low hopping rate as shown below. We propose that the rooted hydroxyl group, which is known to increase the electron carrier density,47–49 is responsible for 10 ACS Paragon Plus Environment

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providing excess carriers that give rise to the high dark current. As shown in Figure 5C and Figure 5D, applying a vacuum or nitrogen atmosphere (1 atm) quickly causes a severe but reversible change in conduction in the dark. Low conductivity in the presence of nitrogen is due to the “natural” conduction in the QD film, essentially hopping conduction that occurs without the increase in free carrier density caused by the rooted hydroxyl groups on the QD surface. To emphasize this effect, the temperature dependence of the dark film conductivity in nitrogen is shown in Figure 5E. Between 100 K and 200 K the conductivity proceeds via Mott variable range hopping (VRH).58 As the temperature decreases, the dominant mechanism changes to Efros-Shklovskii VRH (ES-VRH)59 at 73 K.60–62 The localisation length for the Mott VRH is estimated at 5 nm which is small relative to the SnTe QD size in the film and therefore implies a low absolute conductivity in the system in this temperature range. At 200 K, there is a clear crossover to a 1/T dependence that is indicative of nearest neighbour hopping (NNH), this is common in QD films and has previously been observed in, among other systems, PbSe, CdSe, CdTe and ZnO.63–66 Fitting this to an Arrhenius fit yields an activation energy of 268 meV, a value that is higher than that reported for similar QD systems67 and so indicates that even at higher temperatures, without the hydroxyl surface states, conduction through the film is severely inhibited. Which is qualitatively to be expected if the effective hopping length not only includes the inter-QD separation but also any SnO2 region on the surface. We can also use the NNH fit to calculate the expected change in current if the photoresponse were due to a heating effect. At room temperature, the observed temperature change during measurement would cause the current to increase by < 0.1 nA (at 10 V), not decrease by a much larger value, and thus cannot be the cause of the detection signal.


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This data also eliminates the chance that a pure SnO2 system is responsible for the observed behaviour since tin oxide at room temperature is known to have a higher conductivity under nitrogen than in air68 and displays a positive correlation of conductivity with temperature. This is supported by the lack of photodetection from pure SnO2 NP films in the same device geometry. All these observations show that the underlying QD scaffold is vital to the function of these devices and that the thin oxidised layer on the surface acts only as a hydroxyl bonding site that creates a measurable change in free carrier density in the conduction band of the SnTe core nanocrystals. It is interesting, from a technological standpoint, that these detectors do not respond to visible light. This is demonstrated in Figure 4, where Figure 4B shows the operation of an HgTe device that is otherwise identical to the SnTe device. Similarly to previously described devices,2 when the 670 nm laser diode (LD) is activated it causes a photocurrent in the HgTe device. Even the latest narrowband HgSe devices have interband absorptions at higher energies and thus require the use of an external longpass filter.9 In contrast the operation of the laser diode does not affect the behaviour of the SnTe device (Figure 4C) to incident IR light and thus an imaging device based on these SnTe detectors could not be blinded by visible wavelengths. Conclusion We have shown that the MIR absorbing states of surface species on SnTe and PbSnTe QDs can be used to detect MIR radiation at room temperature and with exposure to air during synthesis and fabrication. These simple planar devices display responsivities up to 6 mA/W at high bias and 2 mA/W below 5 V, to purely MIR radiation, and are resistant to exposure from higher energy light sources. From fitting the data to the known behaviour of SnO2 surfaces it appears that the molecular vibration of an adsorbed hydroxyl species is responsible for the detection of 12 ACS Paragon Plus Environment

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the MIR light. It is definitely the case that the behaviour presented here is a result of the surface chemistry on individual QDs within the film and thus represents one of the first applications of reversible surface doping in QD detector systems. The proposed mechanism of free carriers caused by adsorption of hydroxyl species to tin oxide sites is self-consistent with all observed data, is based on the well understood behaviours of tin surfaces and fits qualitatively within a hopping model. As such this represents the first verified observation of MIR detection using SnTe and PbSnTe QDs. With further refinement in the device performance, these materials could replace mercury nanocrystal based detectors in applications where toxicity is a major concern but the speed of imaging is not. Ultimately though it may be the light activated electrical sensing of a surface species, the mechanism for MIR sensing in SnTe, that makes this system uniquely interesting for further study.


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Methods A more thorough experimental method can be found in the SI. All QDs were synthesized by hot injection method using a co-ordinating solvent and bis[bis(trimethylsilyl)amino]tin(II) as the Sn precursor. Growth was terminated by rapid cooling and the capping agent used was oleic acid (OA). TEM samples were placed on Agar 300 mesh Cu on Formvar grids. TEM, STEM and EDS measurements were done on a JEOL-2100 TEM. Absorbance measurements were done on a Bruker Tensor 27 FTIR with a diamond ATR attachment and a VISNIRMIR Bruker Vertex 80v. All film measurements were done on 1 mm thick CaF2 substrates purchased from Eskma Optics. Device substrates were Si/SiO2 wafers with 300 nm oxide layers with Cr/Au (5/50 nm) interdigitated electrodes (IDE) (36 fingers per side, 2 mm x 20 µm with 10 µm spacing, device area is 4 x 10-6 m2). Electrical measurements were completed on a Keithley SCS-4200 parameter analyzer with the device in ambient atmosphere in the sample chamber of an uncooled Janis VNF-100 optical cryostat. Both the mount and the chamber were monitored for temperature change during measurements. With optical filters in place the maximum recorded temperature change during measurement was 0.05 K / min. Responsivity was calculated as the difference between the current during exposure and the current level to which the device returned once the IR radiation was removed, in order to avoid errors arising from film degradation. Non IR illumination was provided by a Thorlabs 670 nm laser diode, any change in current upon illumination was less than the signal noise of the device. Current vs time measurements were 14 ACS Paragon Plus Environment

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taken with an effective sampling rate of 15 ms and a constantly applied bias. Temperature dependent conductivity measurements were taken with the same setup and filling the sample chamber with liquid nitrogen vapour to the required temperature. SEM images were taken on a JEOL 6500 SEM, before imaging the device was carbon coated to reduce charging. X-Ray diffraction patterns were recorded by a Panalytical X-Ray diffractometer using Cu-Kα radiation, with an operating voltage of 45 kV and current of 40 mA.


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Figure 1. Basic characterisation. A, absorption of the CQD solutions as synthesized, drop-cast and dried on the ATR module of an FTIR. B, NIR-MIR absorption of ligand exchange treated films on CaF2 substrates. Normalised to the absorption at 12000 cm-1, all measurements were done in air and the CO2 absorption at 2400 cm-1 has been removed. The remaining noise areas are due to atmospheric H2O in the optical path. There is a clear absorption at 2.95 µm, except in the PbTe samples, this is marked with the red dashed line in A and B. C, cartoon of device structure. D, SEM image of the top surface and cross section of the device at an electrode, the corresponding EDS map for this area can be found in Figure S12 and S13.


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Figure 2. Devices. A, IV curves, when exposed to only MIR excitation all samples demonstrate a reduction in current. The solid line is measured after the dashed line in every instance. B, Photocurrent as % of dark current for the 4 devices. The PbSnTe device shows the largest percentage change with illumination. C, representative IV curves for 4 different levels of incident IR flux on a PbSnTe device. Blackbody source was at 1073 K. Inset is the current vs time plot when the 3 – 6 µm signal is chopped at 4 Hz. D, shows the calculated responsivity with bias for exposure to a 1073 K BB spectrally cut to 3 – 6 µm, i.e. all of the signal is caused by MIR radiation.


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Figure 3. Investigation of absorption. A, ATR spectra, y-separated, of drop-cast PbSnTe QDs with pure SnO2 and OLA/OA mix shown for reference with the key vibrational modes annotated. B, absorbance of films on CaF2 with various post deposition treatments. C, As2S3 treated film of QDs, showing effect of D2O exposure that replaces some OH groups, with OD groups.


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Figure 4. Cartoon of mechanism for photodetection. A, 1) SnO2 rich surface in the presence of homolytically disassociated atmospheric water. The lone H bonds with a rooted lattice oxygen, forming the rooted OH group. The remaining associated OH group then forms a weak dipole with the lattice Sn.47,48 2) The rooted OH group is ionized and donates an electron to the conduction band in the surface region, which can contribute to hopping transport. 3) The molecular vibration of the rooted OH+ resonates with incident MIR radiation, vibronic coupling occurs between the vibrational state and the bulk electrical state.28 This creates a new state that behaves as an electron trap. 4) An electron from the conduction band is trapped, reducing the carriers available for hopping, thus the bulk observable effect of incident MIR light is a reduction in measured current at constant voltage. B, The operation of a conventional HgTe CQD MIR detector at 1 V bias, the 670 nm laser diode (LD) causes a photocurrent that can blind the detector electronics at intensities above threshold. C, The SnTe device operated at 1 V bias. Although slower than the HgTe device in this configuration, it displays an inverse photoresponse that is unaffected by the laser diode. 19 ACS Paragon Plus Environment

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Figure 5. Investigation of mechanism. A, NIR-MIR absorbance of same PbSnTe film in air and vacuum. B, the MIR absorbance of the same SnTe film in air and in vacuum, for both A and B, aside from the expected atmospheric H2O and CO2 changes in the path, the absorbance of the films fit except for in the region of interest (3000 – 3500 cm-1). The vacuum has clearly changed the absorbance of the film. C, electrical behaviour of the device over time as a vacuum is applied, 10 V bias. D, electrical behaviour of the device over time as N2 is slowly let in to the chamber, this is achieved using a needle valve on the cryostat, 10 V bias. E, conductivity dependence on temperature for the film under nitrogen (1 atm) where SnTe acts as a typical QD film with a Mott-VRH regime below 200 K and 1/T dependent NNH regime above 200 K.


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ASSOCIATED CONTENT Supporting Information. The supporting information contains the following information. SA. XRD SB. Ligand exchange SC. TEM of QDs SD. STEM Mapping SE. SEM EDS Mapping SF. TEM EDS and ICP SG. Time response of device SH. STEM EDS of Te NW / QD blend SI. Synthesis details and full experimental method SJ. PbSnTe synthesis reliability and feature position The following files are available free of charge via the Internet at https://pubs.acs.org Supporting Information (.docx)

AUTHOR INFORMATION Corresponding Author Prof Jonathan E Halpert [email protected] Present Address Prof. Jonathan E. Halpert, Assistant Professor, Department of Chemistry, Hong Kong University of Science & Technology (HKUST)


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources J.E.H. acknowledges the Royal Society of New Zealand for funding under the Marsden Fund via Grant E2646/3416. J.E.H. and M.E.C. also acknowledge funding from the Rutherford Discovery Fellowship via Grant E2675/2990. Acknowledgement J.E.H. acknowledges the Royal Society of New Zealand for funding under the Marsden Fund via Grant E2646/3416. J.E.H. and M.E.C. also acknowledge funding from the Rutherford Discovery Fellowship via Grant E2675/2990. Special thanks to, the Plank Group at SCPS for training and relevant discussions, Prof Bob Buckley at the Robinson Research Institute for training and equipment access and Dr Bruce Charlier of the School of Geography, Environment and Earth Sciences for the ICP-MS measurements.


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