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Hot carrier devices using visible and NIR responsive titanium nitride nanostructures with stoichiometry variation
Optical Materials 97 (2019) 109379
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Hot carrier devices using visible and NIR responsive titanium nitride nanostructures with stoichiometry variation
T
Santanu Podder, Arup R. Pal∗ Plasma Nanotech Laboratory, Physical Sciences Division, Institute of Advanced Study in Science and Technology, Paschim Boragaon, Garchuk, Guwahati, 781035, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Plasmon Tuning of LSPR Hot carrier Photodetector Near-infrared
In this study, we have synthesized spectrally tuneable titanium nitride (TiN) nanostructures as an alternative to conventional plasmonic materials by pulsed DC reactive magnetron sputtering. With the variation of the stoichiometry of the samples as well as by changing size, and distribution of TiN nanoparticles, tuning of Localised Surface Plasmon Resonance (LSPR) band from visible to near-infrared (NIR) region is observed where stoichiometry plays the major role. These nanostructures are incorporated in photodetector device geometry and photocurrent generated by the hot holes in TiN/CuO system is measured. Two devices – one operating in the visible region and the other operating in the NIR region are fabricated. Photoelectrical study reveals that the photo-induced current is generated by the hot carriers which are produced through the nonradiative decay of plasmons occurring on the surface of TiN nanoparticles. This study demonstrates that there is a strong prospect of plasmonic TiN nanoparticle-based infrared photodetector which may overcome the limitations of conventional infrared detectors e.g. requirement of expensive narrow band-gap semiconductors and other associated issues.
1. Introduction Light harvesting through hot carrier generation from the decay of surface plasmon in the plasmonic metal nanostructures has reached a new height in recent years [1–5]. Surface plasmons occur at the interface of a material exhibiting positive real part of their relative permittivity (e.g. vacuum, air, glass and other dielectrics) and a material whose real part of permittivity is negative at a given frequency of light, typically a metal or a heavily doped semiconductor. A strong confinement of electromagnetic field to scales far below that of conventional optics occurs through Localised Surface Plasmon Resonance (LSPR) [6,7] which is the prime reason for all the optoelectronic application of plasmonic materials. The position, intensity, and width of the LSPR band depend on the materials of the used nanostructure, their size, shape as well as on the characteristics of their local dielectric environment [8,9]. Considering the aforementioned facts, the design and fabrication of new plasmonic nanostructures are now of utmost importance for the development of new approaches in the field of plasmonics. Hot carriers generated from the decay of plasmon have a wide range of applications in photo-voltaics [10–16], photo-catalysis [17–19], photo-detection [20,21], solar water splitting [22] etc. Conventional semiconductor-based optoelectronic device architectures associated ∗
with properly designed plasmonic nanostructures can enhance the performance of the device to a great extent because plasmonic materials have the capability to absorb light of energy less than the band gap of the corresponding semiconductor. The hot carriers generated from such plasmonic nanostructures can be injected into the semiconductor matrix in three different ways. In conventional plasmon induced hot electron transfer (PHET) – which is also known as internal photoemission, hot carriers generated through Landau damping get transferred to the semiconductor after overcoming the collision amongst themselves [23–25]. In direct metal to semiconductor interfacial charge transfer transition (DICTT) [24–27], electrons are directly exited to the conduction band of the associated semiconductor after exposure to an electromagnetic excitation. A new approach is revealed by Lian et al., which is, plasmon induced metal to semiconductor interfacial charge transfer transition (PICTT) [28]. If there exists a strong coupling between the metal and semiconductor energy levels, plasmon induced hot carriers can be directly transferred to the semiconductor leading to enhancement of the device performance. The particular choice of materials can have a drastic effect on the degree of confinement and propagation distance due to losses. Showing plasmon resonance and generating hot carrier is very much dependent on carrier concentration [17,29]. Transition metal nitrides, especially titanium nitride (TiN), having carrier concentration comparable to gold
Corresponding author. E-mail address: [email protected] (A.R. Pal).
https://doi.org/10.1016/j.optmat.2019.109379 Received 13 July 2019; Received in revised form 9 September 2019; Accepted 10 September 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
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S. Podder and A.R. Pal
(Au) and silver (Ag) [30], has the potential to be an excellent candidate for light energy harvesting in the visible as well as in the infrared region [31–34]. In some particular cases, plasmonic TiN, having low cost, low loss and high oxidation stability, outperforms the conventional plasmonic materials [22,35,36]. Recently one study revealed that the plasmonic property degradation of TiN with temperature is less as compared to Au [37]. Implementation of plasmonic TiN into optoelectronic device geometry has not been done at a very large scale so far. Hussain et al. first successfully fabricated a plasmonic broadband photodetector using TiN where they used plasma polymerized aniline as a charge transport medium [38]. Hot electron excitation from TiN using visible light is also reported in near past [39]. But, it would be a highly fascinating work if TiN could be realized for the fabrication of an infrared photodetector. Infrared detectors have enormous and paramount importance in space research, remote sensing and defense related application such as night vision camera, thermal imaging, infrared homing etc. But the conventional excitonic infrared photodetectors face the problem of lattice heating. That is why this type of detectors must be associated with cryogenic cooling which increases the cost and volume of the device [40,41]. This problem can be overcome by using plasmonic TiN as light absorbing material along with a wide band gap semiconductor. In this study, we have successfully tuned the plasmon absorbance band of TiN from visible to NIR region by varying the stoichiometry of the samples along with changing of size and distribution of the nanoparticles. This tuning is also realized from the theoretical calculation of absorbance spectra by quasi-static approximation. TiN nanostructures having different morphology are synthesized by pulsed DC reactive magnetron sputtering technique by changing the working pressure and deposition time during the synthesis process. Being a dry plasma based process, it has the advantage of forming very stable nanostructures which improve the quality of the fabricated device when incorporated into the device configuration. Using these as synthesized TiN nanoparticles with finely tuned LSPR band, two plasmonic photodetectors have been fabricated - one operating in the visible region and the other operating in the near-infrared region of the electromagnetic spectrum. In these devices, a 40 nm thick CuO thin film is used as the charge transport medium. The hot carriers produced from the nonradiative plasmon decay of TiN nanoparticles are responsible for the generated photocurrent in both the devices, which is evident from the photoelectrical study. The device parameters i.e. response time, responsivity and specific detectivity of both the devices are calculated and a comparative study has been made.
Table 1 Deposition parameters for the synthesis of samples and details of the fabricated devices: (W.P.- working pressure, D.T.- deposition time). Device
Working Region
Charge Transport Medium
Plasmonic Material used
D1
Visible
CuO thin film of thickness 40 nm
D2
Near Infrared
TiN-1 @ W.P.- 1.5 × 10−1mbar D.T.- 15 min TiN-2 @ W.P.- 7.5 × 10−2mbar D.T.- 10 min
film or as nanoparticles, depends on the working pressure. Higher working pressure favors the formation of nanoparticles. So, in this work, the working pressure and the magnetron substrate distance is optimized to deposit TiN nanoparticles. 2.1. Device fabrication Two devices - one operating in the visible region and the other operating in the near-infrared region of the spectrum, are prepared by varying the TiN deposition condition which is given in Table 1. The device fabrication consists of three steps. First of all, we have deposited 40 nm thick copper oxide (CuO) layer on a patterned ITO-coated glass substrate of surface resistivity 70–100 Ω/sq, having dimension (2.54 × 2.54 cm) by proper masking arrangement. Then TiN nanostructures are deposited on CuO layer by reactive magnetron sputtering process followed by deposition of 100 nm thick Al counter electrode by thermal evaporation technique. Al wires of high purity (99.99%, Alfa Aesar) are thermally evaporated at high vacuum (5 × 10−5 mbar) during the thermal evaporation process. The detailed device fabrication process is depicted in Figure S1 (in the supplementary information). 2.2. Characterization The structural characterization of the synthesized titanium nitride samples is performed by X-Ray Diffractometer (D8 Advance, BRUKER AXS, Germany) using CuKα radiation (λ = 1.5406 A0), working at a voltage of 40 KV and current 40 mA. Surface morphology of the prepared samples is obtained from Field Emission Scanning Electron Microscopy (FESEM) (SIGMA VP ZEISS). The optical properties of the samples are studied using a UV–Vis spectrophotometer (UV-2600, Shimadzu, Japan). The X-Ray and Ultraviolet Photoelectron Spectroscopy (XPS and UPS) studies of the samples are performed by ESCALAB Xi+ (Thermo Fisher). Surface of the samples is cleaned by ion beam (energy 500 eV) etching process for 5 s at a pressure of 9 × 10−9 mbar to minimize the impurities present on the top of the surface before XPS measurements are carried out. Transmission Electron Microscopy (TEM) of the synthesized TiN samples deposited on CuO layer on a Cu grid is carried out with the help of JEM 2100F equipment with an accelerating voltage 200 KV and the particle size distribution is calculated with the software ImageJ (National Institute of Health, USA). Dielectric Constants of the samples are extracted from their Reflection Spectra with the help of RefFIT Software. All the photoelectrical measurements are done by a Keithley electrometer (Keithley 6517B) at room temperature. LEDs of different wavelength are used as light sources. Intensity of light is measured by an optical power meter (Newport, 1830-R). A LeCroy LT264 Oscilloscope is used to record the response speeds of the fabricated devices. All the material and electrical measurements are carried out after the exposure of the samples to the atmosphere.
2. Experimental details The experiment is carried out in a cylindrical plasma reactor which is having a water-cooled planar magnetron assembly, where all the materials are synthesized by reactive pulsed DC magnetron sputtering. For the synthesis of TiN nanoparticles, Ti target (purity 99.99%) of diameter 5.08 cm is attached to the magnetron maintaining a gap of 9 cm between the target and the substrate. Before starting the experiment the whole chamber is cleaned by mechanical etching followed by cleaning with acetone (Merck). Substrates were cleaned ultrasonically in 2-propanol (Merck) as well as in deionized water (MilliQ). Before starting the deposition, the chamber was evacuated to the base pressure 3 × 10−5 mbar by a diffusion pump which is associated with a rotary vane pump. N2 gas which acts as a sputtering as well as reactive gas dissociates into ions by the application of pulsed dc power and helps in maintaining the desired working pressure. These high energetic ions then strike the target to eject Ti atoms from the target surface. But the ejected Ti atoms cannot reach directly to the substrate because they lose their energy due to the collision. Then these atoms react with nitrogen to form TiN molecules. These molecules then interact among themselves to form nanostructures [42]. The details of the deposition conditions are given in Table 1. Whether TiN will be deposited as a smooth
3. Result and discussion The structural information and crystal growth direction of the 2
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Fig. 1. FESEM images of (a) TiN-1 and (b) TiN-2; UV–Vis–NIR Spectra of the active materials of the devices (c) D1 and (d) D2; (e) Tuning of LSPR of TiN from Visible to NIR region.
their reflection spectra with a Drude-Lorentz model by Reffit software as shown in Figure S(5,6) (in the supplementary information). Using the value of dielectric constants we calculated the absorbance cross-section of the active materials of the devices using quasi-static approximation and are presented in Figure S7 (in the supplementary information). This theoretical study stands as a strong support for the realization of tuning of LSPR band from visible to NIR region. Stoichiometry plays a very important role in tuning the LSPR of titanium nitride samples. The stoichiometry of the synthesized samples are determined using X-Ray Photoelectron Spectroscopy and is shown in Fig. 2. The presence of oxygen is due to the formation of titanium oxynitride and titanium dioxide phases in the sample due to the presence of residual oxygen inside the chamber [43]. The amount of titanium atom in TiN-1 and TiN-2 is 37.42% and 36.05% respectively while the nitrogen atomic percentage is 2.07% and 32.64% respectively for these samples. Generally, nitrogen rich samples possess comparatively lower carrier concentration and depart from its metallic behavior which shifts the LSPR peak of TiN-2 towards the higher wavelength region. On the other hand, metal rich TiN-1 sample having higher carrier concentration shows LSPR in the visible region of the electromagnetic spectrum [30,31]. So, this change of stoichiometry of nitrogen properly justifies the tuning of LSPR in titanium nitride nanostructures. Ti 2p and N 1s spectra of both the samples obtained from X-ray photoelectron spectroscopy are de-convoluted and is shown in Fig. 3. Spin-orbit coupling split Ti 2p spectra in two components i.e. Ti 2p3/2 and Ti 2p1/2. Decomposition of Ti 2p spectra of both the samples reveal the contribution from Ti–N, Ti–O and Ti–N–O bonds as indicated in Fig. 3(a and c) [44–46]. Relative amounts of Ti–N and Ti–N–O components play a very important role for the understanding of the shifting of plasmon resonance from visible to NIR region. The ratio of Ti–N to Ti–N–O in TiN-1 sample is 0.58 whereas this ratio is 0.40 for TiN-2. That implies the quantity of Ti–N–O is higher in TiN-2 as compared to TiN-1. Because of this relatively high amount of Ti–N–O, TiN-2 exhibits plasmon resonance in the NIR region. This tuning can be more clearly understood by analyzing the N 1s
synthesized TiN samples are analyzed by X-Ray Diffraction (XRD) analysis and is shown in Figure S2 (in the supplementary information). XRD analysis of both the samples shows the growth of titanium nitride in the (200) direction with interplanar spacing 0.20 nm. The surface morphology of the synthesized TiN samples are obtained through FESEM and shown in Fig. 1(a) and (b). Formation of nanoparticles is confirmed for both the samples. The images also reveal a uniform and homogeneous distribution of the nanoparticles across the substrate. From the FESEM images of both the samples, it is clear that the particle size is comparatively bigger for TiN-2 than TiN-1. The exact size, shape and distribution of the nanoparticles for both the samples is determined through TEM analysis and presented in the subsequent section. FESEM image of CuO is shown in Figure S3 (in the supplementary information). Fig. 1(c) and (d) and show the UV–Vis–NIR spectra of the active materials used in device D1 and D2 respectively. The absorption of CuO is represented by the black curves in both the figures. The CuO layer is not only working as a charge carrier transport material but also increases the effective light absorption of both the devices. UV–Vis–NIR spectrum of TiN-1 shown in Fig. 1(c) reveals absorbance peak at visible region (around 570 nm) while the absorbance maximum of TiN-2 is in the NIR region (around 1000 nm) of the electromagnetic spectrum as shown in Fig. 1(d). Both these absorbance peaks are attributed to the Localised Surface Plasmon Resonance (LSPR) of TiN nanoparticles [22,31,38,43]. The active material i.e. the combination of TiN nanoparticles over CuO matrix for both the devices retains the characteristic absorption peaks of TiN-1 and TiN-2 respectively. Taking into consideration the tuneable plasmon absorption of TiN, it is possible to fabricate plasmonic photo devices which can operate in different ranges of the spectrum from visible to near infrared. The broadness of the absorption peaks also helps in working the fabricated devices in a very broad region. Fig. 1(e) shows the tuning of LSPR of TiN from visible to NIR region. This tuning is also shown by the transmittance spectrum of both the samples as shown in Figure S4 in the supplementary information. The dielectric constants of the materials are extracted by fitting 3
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Fig. 2. X-Ray Photoelectron Spectra of (a) TiN-1 and (b) TiN-2 used to calculate the corresponding atomic percentage.
spectrum [36,47–49]. The value of lattice spacing is calculated from the SAED pattern of TiN-1 and TiN-2 as shown in Fig. 4(c) and (f) respectively. The outer two rings of Fig. 4(c) represent (200) and (220) crystalline planes of titanium nitride with d value 0.20 nm and 0.14 nm respectively. Similarly in Fig. 4(f) the outer circular ring is due to (220) crystalline plane of TiN with lattice spacing 0.14 nm. The inner yellow circular ring of both the samples represents (111) lattice plane of underlying CuO layer with interplanar distance 0.23 nm [50–52].
spectra. Ti–N and Ti–N–O, both the phases contribute to the N 1s spectra of the samples as shown in Fig. 3(b and d). The ratio of Ti–N to Ti–N–O in TiN-1 sample is 1.56 whereas the ratio is 0.85 for TiN-2. This means Ti–N–O component dominates over the Ti–N part in TiN-2 sample which leads to the shifting of plasmon resonance towards the NIR region. Overall, we can say that Ti–N rich samples show plasmon resonance in the visible region, whereas Ti–N–O rich samples exhibit plasmon resonance in the NIR region of the electromagnetic spectra. The TEM analysis of the samples also helps to understand the tuning of LSPR from visible to NIR region. Fig. 4(a) shows the TEM morphology of TiN-1 sample which shows plasmon resonance in the visible region as observed from Fig. 1(c). The corresponding size distribution is shown in Fig. 4(b). It is very much clear from the figure that huge number of TiN nanoparticles of almost spherical shape is deposited. From the size distribution, average particle size is calculated and found to be around 6.5 nm. The plasmon absorbance of TiN-2 is in the NIR region of the spectrum. The TEM image and the corresponding size distribution of that sample is shown in Fig. 4(d) and (e) and respectively. From the figures, it is clear that the shape of the particles are spheroid and the arrangement of nanoparticles over the substrate is dense as compared to TiN-1. The size distribution also reveals an uneven size distribution with average particle size around 12 nm. The increased particle size, number density, distorted spherical/spheroid shape and uneven distribution help in tuning the plasmon resonance in the NIR region of the
3.1. Device characterization Taking into consideration the tuneable plasmon absorption of titanium nitride nanostructures, an effort has been made to fabricate photodetectors operating in two different regions of the spectrum, with changing only the properties of a single material. Especially, plasmonic infrared photodetector will have immense importance in the near future because the existing conventional excitonic infrared detectors are facing a huge problem of lattice heating [41]. Two detectors (D1 and D2) are fabricated where the active medium consisting of CuO thin film and TiN nanostructure, is sandwiched in between two electrodes i.e. ITO and Al. Fig. 5 shows the layout of the fabricated devices. In this device architecture, TiN is working as a light absorbing and hot carrier generating material and CuO works as charge carrier transporting medium. Device D1 containing TiN-1 works in the
Fig. 3. Deconvoluted Ti 2p and N 1s X-Ray Photoelectron Spectra of TiN-1 (a and b) and TiN-2 (c and d) - revealing the details of the composition of the sample. 4
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Fig. 4. (a) TEM Morphology (b) Size distribution (c) SAED pattern of TiN-1 nanostructure, (d) TEM Morphology (e) Size distribution (f) SAED pattern of TiN-2 nanostructure.
in the NIR region. The photosensitivity curves follow the trends of UV–Visible absorption spectra of TiN. This clearly proves that the induced photo-current is generated from the decay of plasmons occurring on the TiN nanoparticles. The stability and response speed of the devices are studied by exposing the devices under a time-varying light of intensity 15 mW/cm2 and 1 Hz frequency. The switching between ON and OFF state of both the devices are shown in Fig. 6(c and f). Both the devices show very stable response but the device D1 generates higher photo-voltage as compared to D2. This behavior is consistent with our previous discussion regarding the better performance of D1 than D2. The rise time (τr) and fall time (τd) of the devices D1 and D2 are calculated from the single cycle on-off switching at zero bias recorded with a LeCroy LT264 oscilloscope as shown in Fig. 7(a) and (b) respectively. The rise time is characterized by the time required to increase the amplitude of transient voltage from 10% to 90% of maximum amplitude once the light source is switched ON, while the fall time is represented by the time needed to fall the amplitude from 90% to 10% once the light source is switched OFF [53]. The rise times are found to be 93 m s and 96 m s for devices D1 and D2 respectively and the corresponding fall times are 167 m s and 186 m s respectively. The performance of a photodetector is characterized by its responsivity and specific detectivity. The ratio of electrical output and optical input is represented by the term responsivity (Rλ). The specific detectivity (D*) is a measure of minimum detectable signal strength above the noise level. A comparative study of responsivity and specific detectivity is done for both the devices at −1 V bias and shown in Fig. 7(c) and (d) and for device D1 responsivity and detectivity is maximum at wavelength 570 nm whereas, for device D2, these parameters have highest values at 1050 nm. The nature of the curves further confirms the fact that the photo-current is generated by the hot carriers which are produced through the decay of plasmons occurring in the TiN nanostructures. The maximum responsivity of D1 at 570 nm wavelength is 56 mA/W and for D2, the value of maximum responsivity is 54 mA/W for 1050 nm wavelength. Similarly, the maximum detectivity for D1 and D2 are 7 × 109 Jones and 5 × 109 Jones respectively. This
Fig. 5. The layout of the synthesized Devices.
visible region of the spectrum, while the working region of device D2 containing TiN-2 is near infrared. Fig. 6 shows the comparison of photo-electrical response of the fabricated devices. Fig. 6(a) shows the I–V characteristics of the device D1 under dark and illumination of white light (intensity 15 mW/cm2), whereas Fig. 6(d) represents the results of the same study of device D2. The non-linearity of the curves arises due to the generation of a Schottky barrier in between TiN and CuO layer. It is clear from the figures that the ratio of current density under light to dark mode is higher in device D1 as compared to D2. The energy corresponding to the LSPR band of TiN in device D1 and D2 is around 2.1 eV and 1.3 eV respectively. This low plasmon energy in device D2 is responsible for the deterioration of the performance of the device as the energy of the carriers generated from plasmon decay will also be correspondingly low. Moreover, as the absorption spectrum is very broad for TiN-2, the plasmon energy is distributed among a very broad density of states of the generated carriers [17]. These are the two possible reasons why all the hot carriers do not get sufficient energy to cross the Schottky barrier in case of device D2. The photosensitivity of both the devices at the bias of −1 V is also studied by illuminating the devices with a series of LEDs having wavelength ranging from 400 nm to 1550 nm as shown in Fig. 6(b) and (e) for devices D1 and D2 respectively. The intensity of the light sources is kept fixed at 0.3 mW/cm2. It is clear from Fig. 6(b) that the device D1 shows the highest photosensitivity in the visible region of the spectrum of around 570 nm, while the photosensitivity of device D2 is maximum 5
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Fig. 6. (a) & (d) I–V characteristics, (b) & (e) Photosensitivity, (c) & (f) On-Off switching (at zero bias) of D-1 and D-2 respectively.
difference between the valence band of CuO and TiN-1 Fermi level is around 0.7 eV. Similarly, the energy difference between the conduction band of CuO and the Fermi level of TiN-1 is around 1.3 eV. Plasmons are generated at the fermi surface of TiN when the light of proper frequency impinges on the device. The decaying plasmon then generates hot carriers and the plasmon energy is distributed among hot electrons and holes. Distribution of plasmon energy among electrons and holes are dependent on the choice of material, particle size, shape etc. For some materials such as Au, energy gained by the holes is greater than that of electrons. In some materials such as Ag, plasmon energy is equally distributed between electrons and holes [2]. Recently, through
comparative study again shows slightly better performance of D1 than D2. The working mechanism of the fabricated devices is proposed based on the energy level diagram shown in Fig. 8(a) and (b) for device D1 and D2 respectively. The position of the valence band (VB) of CuO is taken from the literature [52], and the conduction band (CB) position is determined by measuring band gap of CuO (~ 2.0 eV) by using Tauc technique as shown in S8 (in the supplementary information) [54]. The work function of TiN-1 and TiN-2 are 4.2 eV and 4.6 eV respectively, estimated from the Ultraviolet Photoelectron Spectra as shown in Figure S9 (in the supplementary information) [55]. The energy
Fig. 7. Single cycle On-Off switching of (a) D1 and (b) D2; (c) Spectral Responsivity, (d) Specific Detectivity of D1 and D2. 6
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Fig. 8. Schematic energy level diagram describing the possible working mechanism of the fabricated devices.
the first principle calculations, researchers found that plasmon energy is distributed roughly evenly between hot electrons and holes in case of transition metal nitrides [56]. So, in case of TiN-1, the hot holes have sufficient energy (~1.05 eV) to cross the Schottky barrier and they find a suitable path to be injected into the VB of CuO - resulting in charge carrier separation. But, as the energy barrier between CB of CuO and TiN-1 Fermi level is quite high as compared to the energy of hot electrons, it is not very favorable for the electrons to jump to the CB of CuO. As a result of this separation process, photo-generated current flows through the device. We have kept the thickness of the CuO layer comparatively low i.e. 40 nm to minimize the loss due to carrier recombination. Fig. 8(b) shows the energy level diagram of device D2. In this device, the energy barrier between TiN-2 fermi level and valence band of CuO is reduced as compared to D1, on the other hand, the plasmon energy corresponds to LSPR of TiN-2 is also reduced to 1.3 eV. Moreover, this energy is distributed over a broad distribution of the generated carriers. That is why in spite of having low barrier height this device cannot perform better than device D1. From the above discussion, it is clear that both the devices show appreciable photoresponsivity, response speed and photosensitivity, and specific detectivity. For the enhancement of the overall device performance, further optimization of the device architecture is required. However, the main success of this study is the tuning of plasmon absorbance band of TiN from visible to NIR region and implementation of this tuning in device application, which reveals the prospect of applicability of titanium nitride in infrared sensing by plasmon induced charge generation.
Working principle of the fabricated devices is explored with the help of energy level diagram and found that hot holes are responsible for photocurrent generation in both the devices. Further performance enhancement by device architecture engineering and also the tuning of LSPR band towards higher wavelength region may be possible by tailoring the stoichiometry, size, shape and distribution of nanostructures, which will open up a new window for the application of these devices in remote sensing, space research and defense. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work is financially supported by the Institute of Advanced Study in Science and Technology (IASST), Guwahati, Government of India, and Science and Engineering Research Board, DST, Government of India (SERB File No. EMR/2016/007702). The authors also acknowledge SAIF-NEHU, Shillong and CSIR-NEIST, Jorhat and IITGuwahati for providing Transmission Electron Microscopy, Photoelectron Spectroscopy and Reflection measurement facility respectively. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optmat.2019.109379.
4. Conclusion
References
In this study, we have successfully synthesized plasmonic titanium nitride nanostructures of varied stoichiometry, morphology and distribution through reactive pulsed DC magnetron sputtering. We have tuned the LSPR band of TiN from visible to NIR region of the electromagnetic spectrum. The possible reasons for shifting the absorbance band towards the infrared region is discussed on the basis of the stoichiometry of the samples, as well as the size and distribution of the nanoparticles where stoichiometry plays the dominant role. Tuning is also observed by theoretical study of absorbance spectra from quasistatic approximation. These TiN nanostructures are successfully implemented in device architecture having configuration ITO/CuO/TiN/ Al. Two different photodetectors are fabricated – one operating in the visible region and the other operating in the near infrared region. In these devices, TiN is working as a light absorbing and hot carrier generating material through the decay of plasmon occurring on TiN nanostructure and CuO is used as a carrier transport medium. A comparative photo-electrical study of both the devices is carried out.
[1] M.L. Brongersma, N.J. Halas, P. Nordlander, Plasmon-induced hot carrier science and technology, Nat. Nanotechnol. 10 (2015) 25–34. [2] R. Sundararaman, P. Narang, A.S. Jermyn, W.A. Goddard, H.A. Atwater, Theoretical predictions for hot-carrier generation from surface plasmon decay, Nat. Commun. 5 (2014) 5788. [3] P. Christopher, M. Moskovits, Hot charge carrier transmission from plasmonic nanostructures, Annu. Rev. Phys. Chem. 68 (2017) 379–398. [4] A. Manjavacas, J.G. Liu, V. Kulkarni, P. Nordlander, Plasmon induced hot carriers in metallic nanoparticles, ACS Nano 8 (2014) 7630–7638. [5] P. Narang, R. Sundararaman, H.A. Atwater, Plasmonic hot carrier dynamics in solidstate and chemical systems for energy conversion, Nanophotonics 5 (2016) 96. [6] F.J. Beck, A. Polman, K.R. Catchpole, Tunable light trapping for solar cells using localized surface plasmons, J. Appl. Phys. 105 (2009) 114310. [7] S.A. Maier, Plasmonics: Fundamentals and Applications, Springer, New York, 2007. [8] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, The optical properties of metal nanoparticles: the influence of size, shape, and dielectric environment, J. Phys. Chem. B 107 (2003) 668–677. [9] K.S. Lee, M.A. El-Sayed, Gold and silver nanoparticles in sensing and imaging: sensitivity of plasmon response to size, shape, and metal composition, J. Phys. Chem. B 110 (2006) 19220–19225.
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[33] P. Patsalas, N. Kalfagiannis, S. Kassavetis, Optical properties and plasmonic performance of titanium nitride, Materials 8 (2015) 3128–3154. [34] U. Guler, V.M. Shalaev, A. Boltasseva, Nanoparticle plasmonics: going practical with transition metal nitrides, Mater. Today 18 (2015) 227–237. [35] U. Guler, G.V. Naik, A. Boltasseva, V.M. Shalaev, A.V. Kildishev, Performance analysis of nitride alternative plasmonic materials for localized surface plasmon applications, Appl. Phys. B 107 (2012) 285–291. [36] U. Guler, J.C. Ndukaife, G.V. Naik, A.G.A. Nnanna, A.V. Kildishev, V.M. Shalaev, A. Boltasseva, Local heating with lithographically fabricated plasmonic titanium nitride nanoparticles, Nano Lett. 13 (2013) 6078–6083. [37] H. Reddy, U. Guler, Z. Kudyshev, A.V. Kildishev, V.M. Shalaev, A. Boltasseva, Temperature-dependent optical properties of plasmonic titanium nitride thin films, ACS Photonics 4 (2017) 1413–1420. [38] A.A. Hussain, B. Sharma, T. Barman, A.R. Pal, Self-powered broadband photodetector using plasmonic titanium nitride, ACS Appl. Mater. Interfaces 8 (2016) 4258–4265. [39] S. Ishii, S.L. Shinde, W. Jevasuwan, N. Fukata, T. Nagao, Hot electron excitation from titanium nitride using visible light, ACS Photonics 3 (2016) 1552–1557. [40] J. Piotrowski, Infrared detectors-new trends, Acta Phys. Pol., A 87 (1995) 303–316. [41] A. Karim, J.Y. Andersson, Infrared detectors: advances,challenges and new technologies, IOP Conf. Ser. Mater. Sci. Eng. 51 (2013) 012001. [42] D. Gogoi, A.A. Hussain, A.R. Pal, Plasma based synthesis of nanomaterials for development of plasma enhanced infrared responsive optoelectronic device, Plasma Chem. Plasma Process. 39 (2018) 277–292. [43] J. Zhang, T.P. Chen, X.D. Li, Y.C. Liu, Y. Liu, H.Y. Yang, Investigation of localized surface plasmon resonance of TiN nanoparticles in TiNxOy thin films, Opt. Mater. Express 6 (2016) 2422–2433. [44] D. Shah, A. Catellani, H. Reddy, N. Kinsey, V. Shalaev, A. Boltasseva, A. Calzolari, Controlling the plasmonic properties of ultrathin TiN films at the atomic level, ACS Photonics 5 (2018) 2816–2824. [45] U. Guler, S. Suslov, A.V. Kildishev, A. Boltasseva, V.M. Shalaev, Colloidal plasmonic titanium nitride nanoparticles: properties and applications, Nanophotonics 4 (2015) 269–276. [46] L. Braic, N. Vasilantonakis, A. Mihai, I.J.V. Garcia, S. Fearn, B. Zou, N.M. Alford, B. Doiron, R.F. Oulton, S.A. Maier, A.V. Zayats, P.K. Petrov, Titanium oxynitride thin films with tunable double epsilon-near-zero behavior for nanophotonic applications, ACS Appl. Mater. Interfaces 9 (2017) 29857–29862. [47] S.K. Ghosh, T. Pal, Interparticle coupling effect on the surface plasmon resonance of gold nanoparticles: from theory to application, Chem. Rev. 107 (2007) 4797–4862. [48] V. Amendola, R. Pilot, M. Frasconi, O.M. Marago, M.A. Iati, Surface plasmon resonance in gold nanoparticle: a review, J. Phys. Condens. Matter 29 (2017) 203002–203048. [49] X.D. Li, T.P. Chen, Y. Liu, K.C. Leong, Influnce of localized surface plasmon resonance and free electrons on the optical properties of ultrathin Au films: a study of the aggregation effect, Opt. Express 22 (2014) 5124–5132. [50] W. Wang, Z. Liu, Y. Liu, C. Xu, C. Zheng, G. Wang, A simple wet-chemical synthesis and characterization of CuO nanorods, Appl. Phys. A 76 (2003) 417–420. [51] X. Jiang, T. Herricks, Y. Xia, CuO nanowires can be synthesized by heating copper substrates in air, Nano Lett. 2 (2002) 1333–1338. [52] Q. Hong, Y. Cao, J. Xu, H. Lu, J. He, J.L. Sun, Self-powered ultrafast broadband photodetector based on p-n heterojunctions of CuO/Si nanowire array, ACS Appl. Mater. Interfaces 6 (2014) 20887–20894. [53] T. Barman, A.A. Hussain, B. Sharma, A.R. Pal, Plasmonic hot hole generation by interband transition in gold-polyaniline, Sci. Rep. 5 (2015) 18276. [54] N. Ghobadi, Band gap determination using absorption spectrum fitting procedure, Int. Nano Lett. 3 (2013) 1. [55] S.Y. Lee, J. Chang, Y. Kim, H. Lim, H. Jeon, H. Seo, Depth resolved band alignments of ultrathin TiN/ZrO2 and TiN/ZrO2-Al2O3-ZrO2 dynamic random access memory capacitors, Appl. Phys. Lett. 105 (2014) 201603–201605. [56] A. Habib, F. Florio, R. Sundararaman, Hot carrier dynamics in plasmonic transition metal nitrides, J. Opt. 20 (2018) 064001–064009.
[10] K.R. Catchpole, A. Polman, Plasmonic solar cells, Opt. Express 16 (2008) 21793–21800. [11] H.A. Atwater, A. Polman, Plasmonics for improved photovoltaic devices, Nat. Mater. 9 (2010) 205. [12] S. Linic, P. Christopher, D.B. Ingram, Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy, Nat. Mater. 10 (2011) 911–921. [13] I. Thomann, B.A. Pinaud, Z. Chen, B.M. Clemens, T.F. Jaramillo, M.L. Brongersma, Plasmon enhanced solar-to-fuel energy conversion, Nano Lett. 11 (2011) 3440–3446. [14] Y. Zhang, C. Yam, G.C. Schatz, Fundamental limitations to plasmonic hot-carrier solar cells, J. Phys. Chem. Lett. 7 (2016) 1852–1858. [15] S.K. Cushing, A.D. Bristow, N. Wu, Theoretical maximum efficiency of solar energy conversion in plasmonic metal-semiconductor heterojunctions, Phys. Chem. Chem. Phys. 17 (2015) 30013–30022. [16] S.K. Cushing, N. Wu, Progress and perspectives of plasmon-enhanced solar energy conversion, J. Phys. Chem. Lett. 7 (2016) 666–675. [17] C. Clavero, Plasmon-induced hot-electron generation at nanoparticle/metal-oxide interfaces for photovoltaic and photocatalytic devices, Nat. Photonics 8 (2014) 95–103. [18] S.L. Shinde, S. Ishii, T.D. Dao, R.P. Sugavaneshwar, T. Takei, K.K. Nanda, T. Nagao, Enhanced solar light absorption and photoelectrochemical conversion using TiN nanoparticle-incorporated C3N4-C dot sheets, ACS Appl. Mater. Interfaces 10 (2018) 2460–2468. [19] A. Marimuthu, J. Zhang, S. Linic, Tuning selectivity in propylene epoxidation by plasmon mediated photo-switching of Cu oxidation state, Science 339 (2013) 1590–1593. [20] C.C. Chang, Y.D. Sharma, Y.S. Kim, J.A. Bur, R.V. Shenoi, S. Krishna, D. Huang, S.Y. Lin, A surface plasmon enhanced infrared photodetector based on InAs quantum dots, Nano Lett. 10 (2010) 1704–1709. [21] W. Li, J.G. Valentine, Harvesting the loss: surface plasmon-based hot electron photodetection, Nanophotonics 6 (2017) 177–191. [22] A. Naldoni, U. Guler, Z. Wang, M. Marelli, F. Malara, X. Meng, L.V. Besteiro, A.O. Govorov, A.V. Kildishev, A. Boltasseva, V.M. Shalaev, Broadband hot-electron collection for solar water splitting with plasmonic Titanium Nitride, Adv. Optical Mater. 5 (2017) 1601031-11. [23] A.O. Govorov, H. Zhang, Y.K. Gun’ko, Theory of photoinjection of hot plasmonic carriers from metal nanostructures into semiconductors and surface molecules, J. Phys. Chem. C 117 (2013) 16616–16631. [24] K. Wu, W.E. Rodriguez-Cordoba, Y. Yang, T. Lian, Plasmon-induced hot electron transfer from the Au tip to CdS rod in CdS-Au nanoheterostructures, Nano Lett. 13 (2013) 5255–5263. [25] A. Sharma, C. Sharma, B. Bhattacharyya, K. Gambhir, M. Kumar, S. Chand, R. Mehrotra, S. Husale, Plasmon induced ultrafast injection of hot electrons in Au nanoislands grown on CdS film, J. Mater. Chem. C. 5 (2017) 618–626. [26] M.J. Kale, T. Avanesian, H. Xin, J. Yan, P. Christopher, Controlling catalytic selectivity on metal nanoparticles by direct photoexcitation of adsorbate-metal bonds, Nano Lett. 14 (2014) 5405–5412. [27] H. Petek, Photoexcitation of adsorbates on metal surfaces: one-step or three-step, J. Chem. Phys. 137 (2012) 091704–091711. [28] K. Wu, J. Chen, J.R. McBride, T. Lian, Efficient hot-electron transfer by a plasmoninduced interfacial charge-transfer transition, Science 349 (2015) 632–635. [29] J.M. Luther, P.K. Jain, T. Ewers, A.P. Alivisatos, Localized surface plasmon resonances arising from free carriers in doped quantum dots, Nat. Mater. 10 (2011) 361–366. [30] G.V. Naik, J. Kim, A. Boltasseva, Oxides and nitrides as alternative plasmonic materials in the optical range, Opt. Mater. Express 6 (2011) 1090–1099. [31] G.V. Naik, J.L. Schroeder, X. Ni, A.V. Kildishev, T.D. Sands, A. Boltasseva, Titanium nitride as a plasmonic material for visible and near-infrared wavelengths, Opt. Mater. Express 2 (2012) 478–489. [32] M.B. Cortie, J. Giddings, A. Dowd, Optical properties and plasmon resonances of titanium nitride nanostructures, Nanotechnology 21 (2010) 115201–115208.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Hot carrier devices using visible and NIR responsive titanium nitride nanostructures with stoichiometry variation
T
Santanu Podder, Arup R. Pal∗ Plasma Nanotech Laboratory, Physical Sciences Division, Institute of Advanced Study in Science and Technology, Paschim Boragaon, Garchuk, Guwahati, 781035, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Plasmon Tuning of LSPR Hot carrier Photodetector Near-infrared
In this study, we have synthesized spectrally tuneable titanium nitride (TiN) nanostructures as an alternative to conventional plasmonic materials by pulsed DC reactive magnetron sputtering. With the variation of the stoichiometry of the samples as well as by changing size, and distribution of TiN nanoparticles, tuning of Localised Surface Plasmon Resonance (LSPR) band from visible to near-infrared (NIR) region is observed where stoichiometry plays the major role. These nanostructures are incorporated in photodetector device geometry and photocurrent generated by the hot holes in TiN/CuO system is measured. Two devices – one operating in the visible region and the other operating in the NIR region are fabricated. Photoelectrical study reveals that the photo-induced current is generated by the hot carriers which are produced through the nonradiative decay of plasmons occurring on the surface of TiN nanoparticles. This study demonstrates that there is a strong prospect of plasmonic TiN nanoparticle-based infrared photodetector which may overcome the limitations of conventional infrared detectors e.g. requirement of expensive narrow band-gap semiconductors and other associated issues.
1. Introduction Light harvesting through hot carrier generation from the decay of surface plasmon in the plasmonic metal nanostructures has reached a new height in recent years [1–5]. Surface plasmons occur at the interface of a material exhibiting positive real part of their relative permittivity (e.g. vacuum, air, glass and other dielectrics) and a material whose real part of permittivity is negative at a given frequency of light, typically a metal or a heavily doped semiconductor. A strong confinement of electromagnetic field to scales far below that of conventional optics occurs through Localised Surface Plasmon Resonance (LSPR) [6,7] which is the prime reason for all the optoelectronic application of plasmonic materials. The position, intensity, and width of the LSPR band depend on the materials of the used nanostructure, their size, shape as well as on the characteristics of their local dielectric environment [8,9]. Considering the aforementioned facts, the design and fabrication of new plasmonic nanostructures are now of utmost importance for the development of new approaches in the field of plasmonics. Hot carriers generated from the decay of plasmon have a wide range of applications in photo-voltaics [10–16], photo-catalysis [17–19], photo-detection [20,21], solar water splitting [22] etc. Conventional semiconductor-based optoelectronic device architectures associated ∗
with properly designed plasmonic nanostructures can enhance the performance of the device to a great extent because plasmonic materials have the capability to absorb light of energy less than the band gap of the corresponding semiconductor. The hot carriers generated from such plasmonic nanostructures can be injected into the semiconductor matrix in three different ways. In conventional plasmon induced hot electron transfer (PHET) – which is also known as internal photoemission, hot carriers generated through Landau damping get transferred to the semiconductor after overcoming the collision amongst themselves [23–25]. In direct metal to semiconductor interfacial charge transfer transition (DICTT) [24–27], electrons are directly exited to the conduction band of the associated semiconductor after exposure to an electromagnetic excitation. A new approach is revealed by Lian et al., which is, plasmon induced metal to semiconductor interfacial charge transfer transition (PICTT) [28]. If there exists a strong coupling between the metal and semiconductor energy levels, plasmon induced hot carriers can be directly transferred to the semiconductor leading to enhancement of the device performance. The particular choice of materials can have a drastic effect on the degree of confinement and propagation distance due to losses. Showing plasmon resonance and generating hot carrier is very much dependent on carrier concentration [17,29]. Transition metal nitrides, especially titanium nitride (TiN), having carrier concentration comparable to gold
Corresponding author. E-mail address: [email protected] (A.R. Pal).
https://doi.org/10.1016/j.optmat.2019.109379 Received 13 July 2019; Received in revised form 9 September 2019; Accepted 10 September 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
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(Au) and silver (Ag) [30], has the potential to be an excellent candidate for light energy harvesting in the visible as well as in the infrared region [31–34]. In some particular cases, plasmonic TiN, having low cost, low loss and high oxidation stability, outperforms the conventional plasmonic materials [22,35,36]. Recently one study revealed that the plasmonic property degradation of TiN with temperature is less as compared to Au [37]. Implementation of plasmonic TiN into optoelectronic device geometry has not been done at a very large scale so far. Hussain et al. first successfully fabricated a plasmonic broadband photodetector using TiN where they used plasma polymerized aniline as a charge transport medium [38]. Hot electron excitation from TiN using visible light is also reported in near past [39]. But, it would be a highly fascinating work if TiN could be realized for the fabrication of an infrared photodetector. Infrared detectors have enormous and paramount importance in space research, remote sensing and defense related application such as night vision camera, thermal imaging, infrared homing etc. But the conventional excitonic infrared photodetectors face the problem of lattice heating. That is why this type of detectors must be associated with cryogenic cooling which increases the cost and volume of the device [40,41]. This problem can be overcome by using plasmonic TiN as light absorbing material along with a wide band gap semiconductor. In this study, we have successfully tuned the plasmon absorbance band of TiN from visible to NIR region by varying the stoichiometry of the samples along with changing of size and distribution of the nanoparticles. This tuning is also realized from the theoretical calculation of absorbance spectra by quasi-static approximation. TiN nanostructures having different morphology are synthesized by pulsed DC reactive magnetron sputtering technique by changing the working pressure and deposition time during the synthesis process. Being a dry plasma based process, it has the advantage of forming very stable nanostructures which improve the quality of the fabricated device when incorporated into the device configuration. Using these as synthesized TiN nanoparticles with finely tuned LSPR band, two plasmonic photodetectors have been fabricated - one operating in the visible region and the other operating in the near-infrared region of the electromagnetic spectrum. In these devices, a 40 nm thick CuO thin film is used as the charge transport medium. The hot carriers produced from the nonradiative plasmon decay of TiN nanoparticles are responsible for the generated photocurrent in both the devices, which is evident from the photoelectrical study. The device parameters i.e. response time, responsivity and specific detectivity of both the devices are calculated and a comparative study has been made.
Table 1 Deposition parameters for the synthesis of samples and details of the fabricated devices: (W.P.- working pressure, D.T.- deposition time). Device
Working Region
Charge Transport Medium
Plasmonic Material used
D1
Visible
CuO thin film of thickness 40 nm
D2
Near Infrared
TiN-1 @ W.P.- 1.5 × 10−1mbar D.T.- 15 min TiN-2 @ W.P.- 7.5 × 10−2mbar D.T.- 10 min
film or as nanoparticles, depends on the working pressure. Higher working pressure favors the formation of nanoparticles. So, in this work, the working pressure and the magnetron substrate distance is optimized to deposit TiN nanoparticles. 2.1. Device fabrication Two devices - one operating in the visible region and the other operating in the near-infrared region of the spectrum, are prepared by varying the TiN deposition condition which is given in Table 1. The device fabrication consists of three steps. First of all, we have deposited 40 nm thick copper oxide (CuO) layer on a patterned ITO-coated glass substrate of surface resistivity 70–100 Ω/sq, having dimension (2.54 × 2.54 cm) by proper masking arrangement. Then TiN nanostructures are deposited on CuO layer by reactive magnetron sputtering process followed by deposition of 100 nm thick Al counter electrode by thermal evaporation technique. Al wires of high purity (99.99%, Alfa Aesar) are thermally evaporated at high vacuum (5 × 10−5 mbar) during the thermal evaporation process. The detailed device fabrication process is depicted in Figure S1 (in the supplementary information). 2.2. Characterization The structural characterization of the synthesized titanium nitride samples is performed by X-Ray Diffractometer (D8 Advance, BRUKER AXS, Germany) using CuKα radiation (λ = 1.5406 A0), working at a voltage of 40 KV and current 40 mA. Surface morphology of the prepared samples is obtained from Field Emission Scanning Electron Microscopy (FESEM) (SIGMA VP ZEISS). The optical properties of the samples are studied using a UV–Vis spectrophotometer (UV-2600, Shimadzu, Japan). The X-Ray and Ultraviolet Photoelectron Spectroscopy (XPS and UPS) studies of the samples are performed by ESCALAB Xi+ (Thermo Fisher). Surface of the samples is cleaned by ion beam (energy 500 eV) etching process for 5 s at a pressure of 9 × 10−9 mbar to minimize the impurities present on the top of the surface before XPS measurements are carried out. Transmission Electron Microscopy (TEM) of the synthesized TiN samples deposited on CuO layer on a Cu grid is carried out with the help of JEM 2100F equipment with an accelerating voltage 200 KV and the particle size distribution is calculated with the software ImageJ (National Institute of Health, USA). Dielectric Constants of the samples are extracted from their Reflection Spectra with the help of RefFIT Software. All the photoelectrical measurements are done by a Keithley electrometer (Keithley 6517B) at room temperature. LEDs of different wavelength are used as light sources. Intensity of light is measured by an optical power meter (Newport, 1830-R). A LeCroy LT264 Oscilloscope is used to record the response speeds of the fabricated devices. All the material and electrical measurements are carried out after the exposure of the samples to the atmosphere.
2. Experimental details The experiment is carried out in a cylindrical plasma reactor which is having a water-cooled planar magnetron assembly, where all the materials are synthesized by reactive pulsed DC magnetron sputtering. For the synthesis of TiN nanoparticles, Ti target (purity 99.99%) of diameter 5.08 cm is attached to the magnetron maintaining a gap of 9 cm between the target and the substrate. Before starting the experiment the whole chamber is cleaned by mechanical etching followed by cleaning with acetone (Merck). Substrates were cleaned ultrasonically in 2-propanol (Merck) as well as in deionized water (MilliQ). Before starting the deposition, the chamber was evacuated to the base pressure 3 × 10−5 mbar by a diffusion pump which is associated with a rotary vane pump. N2 gas which acts as a sputtering as well as reactive gas dissociates into ions by the application of pulsed dc power and helps in maintaining the desired working pressure. These high energetic ions then strike the target to eject Ti atoms from the target surface. But the ejected Ti atoms cannot reach directly to the substrate because they lose their energy due to the collision. Then these atoms react with nitrogen to form TiN molecules. These molecules then interact among themselves to form nanostructures [42]. The details of the deposition conditions are given in Table 1. Whether TiN will be deposited as a smooth
3. Result and discussion The structural information and crystal growth direction of the 2
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Fig. 1. FESEM images of (a) TiN-1 and (b) TiN-2; UV–Vis–NIR Spectra of the active materials of the devices (c) D1 and (d) D2; (e) Tuning of LSPR of TiN from Visible to NIR region.
their reflection spectra with a Drude-Lorentz model by Reffit software as shown in Figure S(5,6) (in the supplementary information). Using the value of dielectric constants we calculated the absorbance cross-section of the active materials of the devices using quasi-static approximation and are presented in Figure S7 (in the supplementary information). This theoretical study stands as a strong support for the realization of tuning of LSPR band from visible to NIR region. Stoichiometry plays a very important role in tuning the LSPR of titanium nitride samples. The stoichiometry of the synthesized samples are determined using X-Ray Photoelectron Spectroscopy and is shown in Fig. 2. The presence of oxygen is due to the formation of titanium oxynitride and titanium dioxide phases in the sample due to the presence of residual oxygen inside the chamber [43]. The amount of titanium atom in TiN-1 and TiN-2 is 37.42% and 36.05% respectively while the nitrogen atomic percentage is 2.07% and 32.64% respectively for these samples. Generally, nitrogen rich samples possess comparatively lower carrier concentration and depart from its metallic behavior which shifts the LSPR peak of TiN-2 towards the higher wavelength region. On the other hand, metal rich TiN-1 sample having higher carrier concentration shows LSPR in the visible region of the electromagnetic spectrum [30,31]. So, this change of stoichiometry of nitrogen properly justifies the tuning of LSPR in titanium nitride nanostructures. Ti 2p and N 1s spectra of both the samples obtained from X-ray photoelectron spectroscopy are de-convoluted and is shown in Fig. 3. Spin-orbit coupling split Ti 2p spectra in two components i.e. Ti 2p3/2 and Ti 2p1/2. Decomposition of Ti 2p spectra of both the samples reveal the contribution from Ti–N, Ti–O and Ti–N–O bonds as indicated in Fig. 3(a and c) [44–46]. Relative amounts of Ti–N and Ti–N–O components play a very important role for the understanding of the shifting of plasmon resonance from visible to NIR region. The ratio of Ti–N to Ti–N–O in TiN-1 sample is 0.58 whereas this ratio is 0.40 for TiN-2. That implies the quantity of Ti–N–O is higher in TiN-2 as compared to TiN-1. Because of this relatively high amount of Ti–N–O, TiN-2 exhibits plasmon resonance in the NIR region. This tuning can be more clearly understood by analyzing the N 1s
synthesized TiN samples are analyzed by X-Ray Diffraction (XRD) analysis and is shown in Figure S2 (in the supplementary information). XRD analysis of both the samples shows the growth of titanium nitride in the (200) direction with interplanar spacing 0.20 nm. The surface morphology of the synthesized TiN samples are obtained through FESEM and shown in Fig. 1(a) and (b). Formation of nanoparticles is confirmed for both the samples. The images also reveal a uniform and homogeneous distribution of the nanoparticles across the substrate. From the FESEM images of both the samples, it is clear that the particle size is comparatively bigger for TiN-2 than TiN-1. The exact size, shape and distribution of the nanoparticles for both the samples is determined through TEM analysis and presented in the subsequent section. FESEM image of CuO is shown in Figure S3 (in the supplementary information). Fig. 1(c) and (d) and show the UV–Vis–NIR spectra of the active materials used in device D1 and D2 respectively. The absorption of CuO is represented by the black curves in both the figures. The CuO layer is not only working as a charge carrier transport material but also increases the effective light absorption of both the devices. UV–Vis–NIR spectrum of TiN-1 shown in Fig. 1(c) reveals absorbance peak at visible region (around 570 nm) while the absorbance maximum of TiN-2 is in the NIR region (around 1000 nm) of the electromagnetic spectrum as shown in Fig. 1(d). Both these absorbance peaks are attributed to the Localised Surface Plasmon Resonance (LSPR) of TiN nanoparticles [22,31,38,43]. The active material i.e. the combination of TiN nanoparticles over CuO matrix for both the devices retains the characteristic absorption peaks of TiN-1 and TiN-2 respectively. Taking into consideration the tuneable plasmon absorption of TiN, it is possible to fabricate plasmonic photo devices which can operate in different ranges of the spectrum from visible to near infrared. The broadness of the absorption peaks also helps in working the fabricated devices in a very broad region. Fig. 1(e) shows the tuning of LSPR of TiN from visible to NIR region. This tuning is also shown by the transmittance spectrum of both the samples as shown in Figure S4 in the supplementary information. The dielectric constants of the materials are extracted by fitting 3
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Fig. 2. X-Ray Photoelectron Spectra of (a) TiN-1 and (b) TiN-2 used to calculate the corresponding atomic percentage.
spectrum [36,47–49]. The value of lattice spacing is calculated from the SAED pattern of TiN-1 and TiN-2 as shown in Fig. 4(c) and (f) respectively. The outer two rings of Fig. 4(c) represent (200) and (220) crystalline planes of titanium nitride with d value 0.20 nm and 0.14 nm respectively. Similarly in Fig. 4(f) the outer circular ring is due to (220) crystalline plane of TiN with lattice spacing 0.14 nm. The inner yellow circular ring of both the samples represents (111) lattice plane of underlying CuO layer with interplanar distance 0.23 nm [50–52].
spectra. Ti–N and Ti–N–O, both the phases contribute to the N 1s spectra of the samples as shown in Fig. 3(b and d). The ratio of Ti–N to Ti–N–O in TiN-1 sample is 1.56 whereas the ratio is 0.85 for TiN-2. This means Ti–N–O component dominates over the Ti–N part in TiN-2 sample which leads to the shifting of plasmon resonance towards the NIR region. Overall, we can say that Ti–N rich samples show plasmon resonance in the visible region, whereas Ti–N–O rich samples exhibit plasmon resonance in the NIR region of the electromagnetic spectra. The TEM analysis of the samples also helps to understand the tuning of LSPR from visible to NIR region. Fig. 4(a) shows the TEM morphology of TiN-1 sample which shows plasmon resonance in the visible region as observed from Fig. 1(c). The corresponding size distribution is shown in Fig. 4(b). It is very much clear from the figure that huge number of TiN nanoparticles of almost spherical shape is deposited. From the size distribution, average particle size is calculated and found to be around 6.5 nm. The plasmon absorbance of TiN-2 is in the NIR region of the spectrum. The TEM image and the corresponding size distribution of that sample is shown in Fig. 4(d) and (e) and respectively. From the figures, it is clear that the shape of the particles are spheroid and the arrangement of nanoparticles over the substrate is dense as compared to TiN-1. The size distribution also reveals an uneven size distribution with average particle size around 12 nm. The increased particle size, number density, distorted spherical/spheroid shape and uneven distribution help in tuning the plasmon resonance in the NIR region of the
3.1. Device characterization Taking into consideration the tuneable plasmon absorption of titanium nitride nanostructures, an effort has been made to fabricate photodetectors operating in two different regions of the spectrum, with changing only the properties of a single material. Especially, plasmonic infrared photodetector will have immense importance in the near future because the existing conventional excitonic infrared detectors are facing a huge problem of lattice heating [41]. Two detectors (D1 and D2) are fabricated where the active medium consisting of CuO thin film and TiN nanostructure, is sandwiched in between two electrodes i.e. ITO and Al. Fig. 5 shows the layout of the fabricated devices. In this device architecture, TiN is working as a light absorbing and hot carrier generating material and CuO works as charge carrier transporting medium. Device D1 containing TiN-1 works in the
Fig. 3. Deconvoluted Ti 2p and N 1s X-Ray Photoelectron Spectra of TiN-1 (a and b) and TiN-2 (c and d) - revealing the details of the composition of the sample. 4
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Fig. 4. (a) TEM Morphology (b) Size distribution (c) SAED pattern of TiN-1 nanostructure, (d) TEM Morphology (e) Size distribution (f) SAED pattern of TiN-2 nanostructure.
in the NIR region. The photosensitivity curves follow the trends of UV–Visible absorption spectra of TiN. This clearly proves that the induced photo-current is generated from the decay of plasmons occurring on the TiN nanoparticles. The stability and response speed of the devices are studied by exposing the devices under a time-varying light of intensity 15 mW/cm2 and 1 Hz frequency. The switching between ON and OFF state of both the devices are shown in Fig. 6(c and f). Both the devices show very stable response but the device D1 generates higher photo-voltage as compared to D2. This behavior is consistent with our previous discussion regarding the better performance of D1 than D2. The rise time (τr) and fall time (τd) of the devices D1 and D2 are calculated from the single cycle on-off switching at zero bias recorded with a LeCroy LT264 oscilloscope as shown in Fig. 7(a) and (b) respectively. The rise time is characterized by the time required to increase the amplitude of transient voltage from 10% to 90% of maximum amplitude once the light source is switched ON, while the fall time is represented by the time needed to fall the amplitude from 90% to 10% once the light source is switched OFF [53]. The rise times are found to be 93 m s and 96 m s for devices D1 and D2 respectively and the corresponding fall times are 167 m s and 186 m s respectively. The performance of a photodetector is characterized by its responsivity and specific detectivity. The ratio of electrical output and optical input is represented by the term responsivity (Rλ). The specific detectivity (D*) is a measure of minimum detectable signal strength above the noise level. A comparative study of responsivity and specific detectivity is done for both the devices at −1 V bias and shown in Fig. 7(c) and (d) and for device D1 responsivity and detectivity is maximum at wavelength 570 nm whereas, for device D2, these parameters have highest values at 1050 nm. The nature of the curves further confirms the fact that the photo-current is generated by the hot carriers which are produced through the decay of plasmons occurring in the TiN nanostructures. The maximum responsivity of D1 at 570 nm wavelength is 56 mA/W and for D2, the value of maximum responsivity is 54 mA/W for 1050 nm wavelength. Similarly, the maximum detectivity for D1 and D2 are 7 × 109 Jones and 5 × 109 Jones respectively. This
Fig. 5. The layout of the synthesized Devices.
visible region of the spectrum, while the working region of device D2 containing TiN-2 is near infrared. Fig. 6 shows the comparison of photo-electrical response of the fabricated devices. Fig. 6(a) shows the I–V characteristics of the device D1 under dark and illumination of white light (intensity 15 mW/cm2), whereas Fig. 6(d) represents the results of the same study of device D2. The non-linearity of the curves arises due to the generation of a Schottky barrier in between TiN and CuO layer. It is clear from the figures that the ratio of current density under light to dark mode is higher in device D1 as compared to D2. The energy corresponding to the LSPR band of TiN in device D1 and D2 is around 2.1 eV and 1.3 eV respectively. This low plasmon energy in device D2 is responsible for the deterioration of the performance of the device as the energy of the carriers generated from plasmon decay will also be correspondingly low. Moreover, as the absorption spectrum is very broad for TiN-2, the plasmon energy is distributed among a very broad density of states of the generated carriers [17]. These are the two possible reasons why all the hot carriers do not get sufficient energy to cross the Schottky barrier in case of device D2. The photosensitivity of both the devices at the bias of −1 V is also studied by illuminating the devices with a series of LEDs having wavelength ranging from 400 nm to 1550 nm as shown in Fig. 6(b) and (e) for devices D1 and D2 respectively. The intensity of the light sources is kept fixed at 0.3 mW/cm2. It is clear from Fig. 6(b) that the device D1 shows the highest photosensitivity in the visible region of the spectrum of around 570 nm, while the photosensitivity of device D2 is maximum 5
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Fig. 6. (a) & (d) I–V characteristics, (b) & (e) Photosensitivity, (c) & (f) On-Off switching (at zero bias) of D-1 and D-2 respectively.
difference between the valence band of CuO and TiN-1 Fermi level is around 0.7 eV. Similarly, the energy difference between the conduction band of CuO and the Fermi level of TiN-1 is around 1.3 eV. Plasmons are generated at the fermi surface of TiN when the light of proper frequency impinges on the device. The decaying plasmon then generates hot carriers and the plasmon energy is distributed among hot electrons and holes. Distribution of plasmon energy among electrons and holes are dependent on the choice of material, particle size, shape etc. For some materials such as Au, energy gained by the holes is greater than that of electrons. In some materials such as Ag, plasmon energy is equally distributed between electrons and holes [2]. Recently, through
comparative study again shows slightly better performance of D1 than D2. The working mechanism of the fabricated devices is proposed based on the energy level diagram shown in Fig. 8(a) and (b) for device D1 and D2 respectively. The position of the valence band (VB) of CuO is taken from the literature [52], and the conduction band (CB) position is determined by measuring band gap of CuO (~ 2.0 eV) by using Tauc technique as shown in S8 (in the supplementary information) [54]. The work function of TiN-1 and TiN-2 are 4.2 eV and 4.6 eV respectively, estimated from the Ultraviolet Photoelectron Spectra as shown in Figure S9 (in the supplementary information) [55]. The energy
Fig. 7. Single cycle On-Off switching of (a) D1 and (b) D2; (c) Spectral Responsivity, (d) Specific Detectivity of D1 and D2. 6
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Fig. 8. Schematic energy level diagram describing the possible working mechanism of the fabricated devices.
the first principle calculations, researchers found that plasmon energy is distributed roughly evenly between hot electrons and holes in case of transition metal nitrides [56]. So, in case of TiN-1, the hot holes have sufficient energy (~1.05 eV) to cross the Schottky barrier and they find a suitable path to be injected into the VB of CuO - resulting in charge carrier separation. But, as the energy barrier between CB of CuO and TiN-1 Fermi level is quite high as compared to the energy of hot electrons, it is not very favorable for the electrons to jump to the CB of CuO. As a result of this separation process, photo-generated current flows through the device. We have kept the thickness of the CuO layer comparatively low i.e. 40 nm to minimize the loss due to carrier recombination. Fig. 8(b) shows the energy level diagram of device D2. In this device, the energy barrier between TiN-2 fermi level and valence band of CuO is reduced as compared to D1, on the other hand, the plasmon energy corresponds to LSPR of TiN-2 is also reduced to 1.3 eV. Moreover, this energy is distributed over a broad distribution of the generated carriers. That is why in spite of having low barrier height this device cannot perform better than device D1. From the above discussion, it is clear that both the devices show appreciable photoresponsivity, response speed and photosensitivity, and specific detectivity. For the enhancement of the overall device performance, further optimization of the device architecture is required. However, the main success of this study is the tuning of plasmon absorbance band of TiN from visible to NIR region and implementation of this tuning in device application, which reveals the prospect of applicability of titanium nitride in infrared sensing by plasmon induced charge generation.
Working principle of the fabricated devices is explored with the help of energy level diagram and found that hot holes are responsible for photocurrent generation in both the devices. Further performance enhancement by device architecture engineering and also the tuning of LSPR band towards higher wavelength region may be possible by tailoring the stoichiometry, size, shape and distribution of nanostructures, which will open up a new window for the application of these devices in remote sensing, space research and defense. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work is financially supported by the Institute of Advanced Study in Science and Technology (IASST), Guwahati, Government of India, and Science and Engineering Research Board, DST, Government of India (SERB File No. EMR/2016/007702). The authors also acknowledge SAIF-NEHU, Shillong and CSIR-NEIST, Jorhat and IITGuwahati for providing Transmission Electron Microscopy, Photoelectron Spectroscopy and Reflection measurement facility respectively. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optmat.2019.109379.
4. Conclusion
References
In this study, we have successfully synthesized plasmonic titanium nitride nanostructures of varied stoichiometry, morphology and distribution through reactive pulsed DC magnetron sputtering. We have tuned the LSPR band of TiN from visible to NIR region of the electromagnetic spectrum. The possible reasons for shifting the absorbance band towards the infrared region is discussed on the basis of the stoichiometry of the samples, as well as the size and distribution of the nanoparticles where stoichiometry plays the dominant role. Tuning is also observed by theoretical study of absorbance spectra from quasistatic approximation. These TiN nanostructures are successfully implemented in device architecture having configuration ITO/CuO/TiN/ Al. Two different photodetectors are fabricated – one operating in the visible region and the other operating in the near infrared region. In these devices, TiN is working as a light absorbing and hot carrier generating material through the decay of plasmon occurring on TiN nanostructure and CuO is used as a carrier transport medium. A comparative photo-electrical study of both the devices is carried out.
[1] M.L. Brongersma, N.J. Halas, P. Nordlander, Plasmon-induced hot carrier science and technology, Nat. Nanotechnol. 10 (2015) 25–34. [2] R. Sundararaman, P. Narang, A.S. Jermyn, W.A. Goddard, H.A. Atwater, Theoretical predictions for hot-carrier generation from surface plasmon decay, Nat. Commun. 5 (2014) 5788. [3] P. Christopher, M. Moskovits, Hot charge carrier transmission from plasmonic nanostructures, Annu. Rev. Phys. Chem. 68 (2017) 379–398. [4] A. Manjavacas, J.G. Liu, V. Kulkarni, P. Nordlander, Plasmon induced hot carriers in metallic nanoparticles, ACS Nano 8 (2014) 7630–7638. [5] P. Narang, R. Sundararaman, H.A. Atwater, Plasmonic hot carrier dynamics in solidstate and chemical systems for energy conversion, Nanophotonics 5 (2016) 96. [6] F.J. Beck, A. Polman, K.R. Catchpole, Tunable light trapping for solar cells using localized surface plasmons, J. Appl. Phys. 105 (2009) 114310. [7] S.A. Maier, Plasmonics: Fundamentals and Applications, Springer, New York, 2007. [8] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, The optical properties of metal nanoparticles: the influence of size, shape, and dielectric environment, J. Phys. Chem. B 107 (2003) 668–677. [9] K.S. Lee, M.A. El-Sayed, Gold and silver nanoparticles in sensing and imaging: sensitivity of plasmon response to size, shape, and metal composition, J. Phys. Chem. B 110 (2006) 19220–19225.
7
Optical Materials 97 (2019) 109379
S. Podder and A.R. Pal
[33] P. Patsalas, N. Kalfagiannis, S. Kassavetis, Optical properties and plasmonic performance of titanium nitride, Materials 8 (2015) 3128–3154. [34] U. Guler, V.M. Shalaev, A. Boltasseva, Nanoparticle plasmonics: going practical with transition metal nitrides, Mater. Today 18 (2015) 227–237. [35] U. Guler, G.V. Naik, A. Boltasseva, V.M. Shalaev, A.V. Kildishev, Performance analysis of nitride alternative plasmonic materials for localized surface plasmon applications, Appl. Phys. B 107 (2012) 285–291. [36] U. Guler, J.C. Ndukaife, G.V. Naik, A.G.A. Nnanna, A.V. Kildishev, V.M. Shalaev, A. Boltasseva, Local heating with lithographically fabricated plasmonic titanium nitride nanoparticles, Nano Lett. 13 (2013) 6078–6083. [37] H. Reddy, U. Guler, Z. Kudyshev, A.V. Kildishev, V.M. Shalaev, A. Boltasseva, Temperature-dependent optical properties of plasmonic titanium nitride thin films, ACS Photonics 4 (2017) 1413–1420. [38] A.A. Hussain, B. Sharma, T. Barman, A.R. Pal, Self-powered broadband photodetector using plasmonic titanium nitride, ACS Appl. Mater. Interfaces 8 (2016) 4258–4265. [39] S. Ishii, S.L. Shinde, W. Jevasuwan, N. Fukata, T. Nagao, Hot electron excitation from titanium nitride using visible light, ACS Photonics 3 (2016) 1552–1557. [40] J. Piotrowski, Infrared detectors-new trends, Acta Phys. Pol., A 87 (1995) 303–316. [41] A. Karim, J.Y. Andersson, Infrared detectors: advances,challenges and new technologies, IOP Conf. Ser. Mater. Sci. Eng. 51 (2013) 012001. [42] D. Gogoi, A.A. Hussain, A.R. Pal, Plasma based synthesis of nanomaterials for development of plasma enhanced infrared responsive optoelectronic device, Plasma Chem. Plasma Process. 39 (2018) 277–292. [43] J. Zhang, T.P. Chen, X.D. Li, Y.C. Liu, Y. Liu, H.Y. Yang, Investigation of localized surface plasmon resonance of TiN nanoparticles in TiNxOy thin films, Opt. Mater. Express 6 (2016) 2422–2433. [44] D. Shah, A. Catellani, H. Reddy, N. Kinsey, V. Shalaev, A. Boltasseva, A. Calzolari, Controlling the plasmonic properties of ultrathin TiN films at the atomic level, ACS Photonics 5 (2018) 2816–2824. [45] U. Guler, S. Suslov, A.V. Kildishev, A. Boltasseva, V.M. Shalaev, Colloidal plasmonic titanium nitride nanoparticles: properties and applications, Nanophotonics 4 (2015) 269–276. [46] L. Braic, N. Vasilantonakis, A. Mihai, I.J.V. Garcia, S. Fearn, B. Zou, N.M. Alford, B. Doiron, R.F. Oulton, S.A. Maier, A.V. Zayats, P.K. Petrov, Titanium oxynitride thin films with tunable double epsilon-near-zero behavior for nanophotonic applications, ACS Appl. Mater. Interfaces 9 (2017) 29857–29862. [47] S.K. Ghosh, T. Pal, Interparticle coupling effect on the surface plasmon resonance of gold nanoparticles: from theory to application, Chem. Rev. 107 (2007) 4797–4862. [48] V. Amendola, R. Pilot, M. Frasconi, O.M. Marago, M.A. Iati, Surface plasmon resonance in gold nanoparticle: a review, J. Phys. Condens. Matter 29 (2017) 203002–203048. [49] X.D. Li, T.P. Chen, Y. Liu, K.C. Leong, Influnce of localized surface plasmon resonance and free electrons on the optical properties of ultrathin Au films: a study of the aggregation effect, Opt. Express 22 (2014) 5124–5132. [50] W. Wang, Z. Liu, Y. Liu, C. Xu, C. Zheng, G. Wang, A simple wet-chemical synthesis and characterization of CuO nanorods, Appl. Phys. A 76 (2003) 417–420. [51] X. Jiang, T. Herricks, Y. Xia, CuO nanowires can be synthesized by heating copper substrates in air, Nano Lett. 2 (2002) 1333–1338. [52] Q. Hong, Y. Cao, J. Xu, H. Lu, J. He, J.L. Sun, Self-powered ultrafast broadband photodetector based on p-n heterojunctions of CuO/Si nanowire array, ACS Appl. Mater. Interfaces 6 (2014) 20887–20894. [53] T. Barman, A.A. Hussain, B. Sharma, A.R. Pal, Plasmonic hot hole generation by interband transition in gold-polyaniline, Sci. Rep. 5 (2015) 18276. [54] N. Ghobadi, Band gap determination using absorption spectrum fitting procedure, Int. Nano Lett. 3 (2013) 1. [55] S.Y. Lee, J. Chang, Y. Kim, H. Lim, H. Jeon, H. Seo, Depth resolved band alignments of ultrathin TiN/ZrO2 and TiN/ZrO2-Al2O3-ZrO2 dynamic random access memory capacitors, Appl. Phys. Lett. 105 (2014) 201603–201605. [56] A. Habib, F. Florio, R. Sundararaman, Hot carrier dynamics in plasmonic transition metal nitrides, J. Opt. 20 (2018) 064001–064009.
[10] K.R. Catchpole, A. Polman, Plasmonic solar cells, Opt. Express 16 (2008) 21793–21800. [11] H.A. Atwater, A. Polman, Plasmonics for improved photovoltaic devices, Nat. Mater. 9 (2010) 205. [12] S. Linic, P. Christopher, D.B. Ingram, Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy, Nat. Mater. 10 (2011) 911–921. [13] I. Thomann, B.A. Pinaud, Z. Chen, B.M. Clemens, T.F. Jaramillo, M.L. Brongersma, Plasmon enhanced solar-to-fuel energy conversion, Nano Lett. 11 (2011) 3440–3446. [14] Y. Zhang, C. Yam, G.C. Schatz, Fundamental limitations to plasmonic hot-carrier solar cells, J. Phys. Chem. Lett. 7 (2016) 1852–1858. [15] S.K. Cushing, A.D. Bristow, N. Wu, Theoretical maximum efficiency of solar energy conversion in plasmonic metal-semiconductor heterojunctions, Phys. Chem. Chem. Phys. 17 (2015) 30013–30022. [16] S.K. Cushing, N. Wu, Progress and perspectives of plasmon-enhanced solar energy conversion, J. Phys. Chem. Lett. 7 (2016) 666–675. [17] C. Clavero, Plasmon-induced hot-electron generation at nanoparticle/metal-oxide interfaces for photovoltaic and photocatalytic devices, Nat. Photonics 8 (2014) 95–103. [18] S.L. Shinde, S. Ishii, T.D. Dao, R.P. Sugavaneshwar, T. Takei, K.K. Nanda, T. Nagao, Enhanced solar light absorption and photoelectrochemical conversion using TiN nanoparticle-incorporated C3N4-C dot sheets, ACS Appl. Mater. Interfaces 10 (2018) 2460–2468. [19] A. Marimuthu, J. Zhang, S. Linic, Tuning selectivity in propylene epoxidation by plasmon mediated photo-switching of Cu oxidation state, Science 339 (2013) 1590–1593. [20] C.C. Chang, Y.D. Sharma, Y.S. Kim, J.A. Bur, R.V. Shenoi, S. Krishna, D. Huang, S.Y. Lin, A surface plasmon enhanced infrared photodetector based on InAs quantum dots, Nano Lett. 10 (2010) 1704–1709. [21] W. Li, J.G. Valentine, Harvesting the loss: surface plasmon-based hot electron photodetection, Nanophotonics 6 (2017) 177–191. [22] A. Naldoni, U. Guler, Z. Wang, M. Marelli, F. Malara, X. Meng, L.V. Besteiro, A.O. Govorov, A.V. Kildishev, A. Boltasseva, V.M. Shalaev, Broadband hot-electron collection for solar water splitting with plasmonic Titanium Nitride, Adv. Optical Mater. 5 (2017) 1601031-11. [23] A.O. Govorov, H. Zhang, Y.K. Gun’ko, Theory of photoinjection of hot plasmonic carriers from metal nanostructures into semiconductors and surface molecules, J. Phys. Chem. C 117 (2013) 16616–16631. [24] K. Wu, W.E. Rodriguez-Cordoba, Y. Yang, T. Lian, Plasmon-induced hot electron transfer from the Au tip to CdS rod in CdS-Au nanoheterostructures, Nano Lett. 13 (2013) 5255–5263. [25] A. Sharma, C. Sharma, B. Bhattacharyya, K. Gambhir, M. Kumar, S. Chand, R. Mehrotra, S. Husale, Plasmon induced ultrafast injection of hot electrons in Au nanoislands grown on CdS film, J. Mater. Chem. C. 5 (2017) 618–626. [26] M.J. Kale, T. Avanesian, H. Xin, J. Yan, P. Christopher, Controlling catalytic selectivity on metal nanoparticles by direct photoexcitation of adsorbate-metal bonds, Nano Lett. 14 (2014) 5405–5412. [27] H. Petek, Photoexcitation of adsorbates on metal surfaces: one-step or three-step, J. Chem. Phys. 137 (2012) 091704–091711. [28] K. Wu, J. Chen, J.R. McBride, T. Lian, Efficient hot-electron transfer by a plasmoninduced interfacial charge-transfer transition, Science 349 (2015) 632–635. [29] J.M. Luther, P.K. Jain, T. Ewers, A.P. Alivisatos, Localized surface plasmon resonances arising from free carriers in doped quantum dots, Nat. Mater. 10 (2011) 361–366. [30] G.V. Naik, J. Kim, A. Boltasseva, Oxides and nitrides as alternative plasmonic materials in the optical range, Opt. Mater. Express 6 (2011) 1090–1099. [31] G.V. Naik, J.L. Schroeder, X. Ni, A.V. Kildishev, T.D. Sands, A. Boltasseva, Titanium nitride as a plasmonic material for visible and near-infrared wavelengths, Opt. Mater. Express 2 (2012) 478–489. [32] M.B. Cortie, J. Giddings, A. Dowd, Optical properties and plasmon resonances of titanium nitride nanostructures, Nanotechnology 21 (2010) 115201–115208.
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