Nanoscale

PAPER

Cite this: DOI: 10.1039/c6nr02579j

Received 28th March 2016,Accepted 22nd June 2016

DOI: 10.1039/c6nr02579j

www.rsc.org/nanoscale

Cation exchange synthesis of uniform PbSe/PbS core/shell tetra-pods and their use asnear-infrared photodetectors†

N. Mishra,*‡a B. Mukherjee,b G. Xing,c S. Chakrabortty,a A. Guchhaita and J. Y. Lima

In this work we explore the preparation of complex-shaped semiconductor nanostructures composed of

different materials via a cationic exchange process in which the cations of the original semiconductor

nanostructure are replaced by cations of different metals with preservation of the shape and the anionic

framework of the nanocrystals. Utilizing this cation exchange method, we synthesized two new tetrapods

for the first time: Cu2−xSe/Cu2−xS and PbSe/PbS, both prepared from CdSe/CdS tetrapods as ‘templates’.

We also fabricated near-infrared (NIR) photodetectors with a very simple architecture comprising a

PbSe/PbS tetrapod layer between two Au electrodes on a glass substrate. When illuminated by a NIR laser,

these devices are capable of achieving a responsivity of 11.9 A W−1 without the use of ligand-exchange

processes, thermal annealing or hybrid device architecture. Transient absorption spectroscopy was

carried out on these PbSe/PbS tetrapods, the results of which suggest that the branched morphology

contributes in part to device performance. Investigation of the charge dynamics of the PbSe/PbS tetra-

pods revealed an extremely long-lived exciton recombination lifetime of ∼17 ms, which can result in

enhanced photoconductive gain. Overall, these heterostructured tetrapods showcase simultaneously the

importance of nanoparticle shape, band structure, and surface chemistry in the attainment of NIR

photodetection.

Introduction

Tetrapod shaped solution-grown core/shell semiconductornanocrystals (NCs) have attracted considerable interest inrecent years because of their many unique properties, includ-ing large absorption cross-sections,1 long biexciton lifetime,2

efficient charge separation3 and increased charge percolationpathways through their arms,4 and their large volume fractionscan accommodate multiexcitons.5 These properties makethem candidates for use in several optoelectronic applicationslike photovoltaics,3,6 lasing media2 and dual electrolumines-

cence.7 To date, core/shell tetrapods have been limited tosemiconductor composition like those belonging to theII–VI group, which include CdSe/CdS,1,8–10 CdSe/CdTe,3,6

CdTe/CdTe,10 ZnTe/CdSe,10,11 ZnTe/CdS,10–12 ZnTe/CdTe10 andZnSe/CdS.10,12

IV–VI semiconductor NCs (e.g. PbS and PbSe) show strongquantum confinement in the infrared (IR) region13 and cansustain multiple exciton generation14,15 and thus are exten-sively investigated for their use in solar cells16,17 andphotodetectors.18–24 PbSe/PbS core shell structures couldpotentially improve these properties because the PbS shell pro-vides protection against oxidation of the PbSe core.25 Addition-ally, facile charge separation occurs due to high dielectricconstants and quasi-type-II band alignment of the core/shellsystem.25,26 Spherical27,28 and wire shaped29 core/shell synth-eses have been reported, but to the best of our knowledge,reports on the synthesis of PbSe/PbS core/shell tetrapods arerare, if not nonexistent. The unique structure of PbSe/PbS tet-rapods could improve the efficiency of photovoltaic and photo-detector devices because of their large linear absorption crosssection. This is due to their tetrahedral geometry, where thearms capture photons from wide angles and polarizationsmore effectively than core/shell dots or nanorods of the samevolume.1,2 Furthermore, reduced spatial overlap between the

†Electronic supplementary information (ESI) available. See DOI:10.1039/c6nr02579j‡Present location: Materials Physics and Applications Division: Center forIntegrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos,New Mexico 87545, USA.

aDepartment of Chemistry, National University of Singapore, 3 Science Drive 3,

Singapore 117543. E-mail: Nimai@lanl.gov, nimaiiitm@gmail.combDepartment of Physics, National University of Singapore, 2 Science Drive 3,

Singapore 117542cKey Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials

(IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials

(SICAM), Nanjing Tech University (Nanjing Tech), 30 South Puzhu Road,

Nanjing 211816, China

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PbSe electron and hole wave functions is expected to yield verylong exciton lifetimes.25 However, the PbSe/PbS tetrapod syn-thesis is challenging due to the difficulty of controlling nuclea-tion and growth rates. The cation exchange process30–34 is anestablished field of research which allows chemical transform-ation by means of substitution of the original cations with newones while preserving the anion framework. The appeal of thiscation exchange method is that one can use traditionally(hot-injection) synthesized nanocrystals with different sizes,shapes, compositions and crystal structures as anion scaffoldsfor the preparation of new materials that currently cannot besynthesized through direct methods.30–32,35 It has beenrecently reported that cation exchange with Pb2+ ions is poss-ible for simple nanocrystal structures including spheres androds36,37 while cation exchange with Cu+ ions was successfulfor both simple NCs and octapods.38 The synthesis of tetra-podal PbSe/PbS and Cu2−xSe/Cu2−xS nanocrystals via a cationexchange reaction has not been reported in the literature.

Photodetection at near-infrared (NIR) wavelengths isbecoming increasingly important due to its use in optical tomo-graphy,39 process monitoring,40 night vision,41 and manymore in a growing list of applications. A potentially economi-cal approach to the fabrication of IR photodetectors is via theuse of solution-processed colloidal semiconductor nanocrys-tals as the active material, given the fact that their spectralrange is readily tunable as a function of particle size, can besynthesized in large quantities, are amenable to a wide rangeof wet-chemical deposition techniques, and can therefore beintegrated with a plethora of different substrates. Owing tothese salient physicochemical properties, NC-based IR photo-detectors have been a very active area of research in recentyears, and NCs based on PbS,19 PbSe,42 Bi2S3,

43 In2S3,44

HgTe,45 HgTe/As2S346 and pyrite FeS2

47 have been explored asthe active material in photodetectors of various architectures.Amongst these materials explored, PbS and PbSe quantumdots are capable of attaining some of the highest NC-based IRdetection efficiencies to date due to long carrier lifetimes of∼0.1–1 µs caused by fortuitous shallow surface traps whichfacilitate charge separation and inhibit recombination.48,49

Further improvements to the detection sensitivity in PbS-baseddevices may be afforded via post synthetic surface modifi-cation, such as ligand exchange with short-chain molecules inorder to reduce interparticle distance within the activematrix.19,50 Other approaches to deriving higher performancein NC-based IR photodetectors may be attributed to the designand fabrication of sophisticated device architectures such asstacked heterojunctions,20,47 or the use of hybrid materialblends such as graphene–NC composites as the activelayer.51,52 While the responsivity and gain attained can be veryhigh, the fabrication of such devices is a multi-step processwhich can be complicated and difficult to optimize due to thelarge number of parameters involved in the final device con-figuration. Another tenable route to the fabrication of efficientIR NC-based photodetectors is the use of branched structuresof lead chalcogenide nanocrystals which has not been studiedearlier.

In this work we report the cation exchange synthesis ofPbSe seeded PbS core/shell tetrapods starting from CdSeseeded CdS tetrapods as a host material via intermediateCu2−xSe seeded Cu2−xS tetrapods. This process works with nointer-diffusion of S and Se elements during the replacement ofCd2+ ions with Cu+ ions and subsequently Cu+ ions with Pb2+

which confirms the conservation of the anionic framework.Our synthetic control retained the shape and size uniformity,monodispersity of the Cu2−xSe/Cu2−xS and PbSe/PbS tetrapods.Subsequently the complete chemical transformation via cationexchange was confirmed by the structural and optical charac-terization. Furthermore we investigate the as synthesized PbSeseeded PbS tetrapods as the active material in a NIR photo-detector device. The tetrapods are incorporated into the devicewithout post-synthetic surface modification while the devicearchitecture is a simple lateral configuration, comprising adropcast tetrapod film sandwiched between two coplanar pre-fabricated Au electrodes supported by a glass substrate. ThesePbSe seeded PbS tetrapod-based photodetectors are capable ofexhibiting a responsivity (Rλ) of up to ∼11.9 AW−1 under broadbeam irradiation (wavelength 808 nm) at a fixed external biasof +4.0 V. Importantly, tetrapod based devices showed anacceptable temporal response of ∼0.98 s rising time and ∼0.1 sdecay time. Transient absorption spectroscopy was carried outto understand the charge recombination dynamics whichrevealed an extremely long-lived exciton lifetime of ∼17 ms,which is highly advantageous for photoconductive gain.Overall, the PbSe seeded PbS tetrapod-based photodetectordescribed in this work is particularly appealing in that it pos-sesses an extremely simple device architecture that is facile tofabricate, yet exhibits acceptable photon detection efficiency atNIR wavelengths.

Results and discussion

The synthesis of PbSe seeded PbS tetrapods proceeded via aseries of cationic exchange processes involving CdSe seededCdS tetrapods as the starting material and Cu2−xSe seededCu2−xS tetrapods as intermediate structures, analogous to aprevious report on the cation exchange synthesis.32,35,37 Sche-matic in Fig. 1a shows our efforts in order to synthesize thosetetrapods with sequential cation exchange. The synthesis ofhighly monodisperse CdSe seeded CdS tetrapods (Fig. 1b) wascarried out via a slight modification of a previously reportedseeded approach.8,9 Briefly, zinc blende CdSe (zb-CdSe) coreswere injected at an elevated temperature into a solvent con-taining Cd and S precursors in the presence of a mixture ofoleic acid and octadecylphosphonic acid (ODPA) as surfac-tants, thus resulting in the heterogeneous growth of CdS armsfrom the CdSe core. While it is clear that the seeded approachdescribed above cannot be used to derive tetrapods from thecubic PbSe core, the conversion of CdSe seeded CdS tetrapodsto PbSe seeded PbS tetrapods via direct exchange with Pb2+

was also not possible, as previously observed by Luther et al.for the cation exchange mediated transformation of CdS to

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PbS nanorods.37 On the other hand, exposure of CdSe seededCdS tetrapods to Cu+ in a mixture of toluene and methanolunder inert conditions results in Cu2−xSe seeded Cu2−xS struc-tures (Fig. 1c) of the same morphology due to the fact that theanionic framework is preserved during the cation-exchangeprocess with Cu+.32 The Energy-dispersive X-ray spectroscopy(EDX) analysis on these tetrapods confirms the elemental com-position, which can be seen in ESI Fig. S3.† Further structuralcharacterization such as X-ray diffraction (XRD) analysis wascarried out on the intermediate Cu2−xSe/Cu2−xS tetrapods (ESIFig. S4†) and this could indicate the presence of Djurleite(JCPDS no. 00-023-0959) crystals in the sample. Subsequentexposure of these tetrapods to excess Pb2+ in the presence oftrioctylphosphine (TOP) yielded uniform PbSe seeded PbS tetra-pod structures, as depicted in Fig. 1d. The arms of the Pb2+

exchanged tetrapods were on average approximately ∼26 nmlong with a diameter of ∼7 nm, which is comparable to thedimensions of the original CdSe seeded CdS tetrapods. Duringthe cation exchange process, the reaction progress could bemonitored due to obvious color changes during the exchangereaction as seen in Fig. 2a. It was observed that during theexchange of Cd2+ with Cu+, the color changed from yellow togolden brown instantaneously. Subsequently, during the Cu+

to Pb2+ exchange, the color changed from golden brown toblack after the addition of TOP.

This suggested that the addition of TOP was the drivingforce for this sequential cation exchange reaction. All pro-cessed tetrapod solutions were clear and free of aggregation

after re-dispersion in tetrachloroethylene. The absorbancespectra for the tetrapods solution are shown in Fig. 2b. Beforethe addition of copper(I) salt for a cation exchange reaction,the absorption spectrum for CdSe/CdS tetrapods clearly showsthe presence of the absorption shoulder at around 500 nmwhich is expected that the absorption profile is essentiallydominated by the four CdS rod-like arms.8 After the cationexchange reaction with copper(I) salt, the spectrum (blue) nolonger retains this absorption feature since there is no absorp-tion contribution from CdS arms. The Cu2−xSe/Cu2−xS tetra-pods show strong plasmonic peaks as compared to theCdSe/CdS tetrapods and this confirms the cation exchangefrom Cd to the Cu system. The optical absorption spectrum(red) of the PbSe seeded PbS tetrapods dispersed intetrachloroethylene was relatively featureless, with a broad exci-tation profile that extended across the NIR spectral window.Fig. 2c shows the effect of left over Cu after Pb exchange onthe absorption profile of PbSe/PbS tetrapods. Three differentcation exchange reactions were performed with varying theamount of precursors. Due to crude approximation of the

Fig. 1 (a) Schematic of the synthesis of the PbSe/PbS tetrapods bysequential cation exchange starting with CdSe/CdS. (b) TEM image ofCdSe/CdS tetrapods before cation exchange, (c) TEM image of Cu2−xSe/Cu2−xS tetrapods after cation exchange and (d) TEM image ofPbSe/PbS tetrapods after sequential cation exchange for 96 hours ofreaction with PbS arm dimensions of ∼26 nm in length and ∼7 nm indiameter.

Fig. 2 (a) Photographs of processed CdSe/CdS, Cu2−xSe/Cu2−xS andPbSe/PbS tetrapod solutions in toluene (left to right) synthesized fromcation exchange reactions. (b) Absorption spectra for CdSe/CdS (black),Cu2−xSe/Cu2−xS (blue) and PbSe/PbS (red) tetrapods in tetrachloroethyl-ene. (c) Absorption spectra for PbSe/PbS tetrapods in tetrachloroethyl-ene solvent with a certain amount of left over Cu. The % of Cu areshown in the figure corresponding to each absorption spectrum. (d)XRD patterns for PbSe/PbS tetrapods after sequential cation exchange.(e) EDX data confirm the full cation exchange and form PbSe/PbStetrapods.

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CdSe/CdS tetrapod’s concentration, certain error occurs duringthe calculation of amount of Cu and Pb precursors and chelat-ing agent TOP which leads to the presence of some amount ofresidual Cu in the final product. Careful EDX analysis of thesePbSe/PbS tetrapods confirms that the presence of plasmonicemission is coming from the left-over Cu after Pb exchange.Elemental quantification was observed from the EDX, whichcan be seen in ESI Fig. S5.† We need to clarify that thesamples used for the device fabrication have a minimumamount of left-over Cu (0–7%). Structural characterization ofthe final PbSe seeded PbS tetrapods was carried out via XRDmeasurements, as depicted in Fig. 2d, and the peaks obtainedwere referenced with those of standard Halite PbS (JCPDS no.00-005-0592). The close agreement with all of the referenceindex planes of PbS suggests that the PbSe seeded PbS tetra-pod arms are highly crystalline even though their formationwas carried out at room temperature, thus attesting to theexquisite conservation of the anionic lattice of the originalcrystalline CdSe/CdS nanostructures despite undergoing twoconsecutive cationic exchange reactions. Further characterizationof the elemental composition of the derived PbSe seeded PbStetrapods proceeded via EDX measurements, as shown inFig. 2e, where the presence of Pb and S is prominently featured.Signals attributed to Se, however, were extremely weak and couldnot be assigned with confidence, likely because the PbSe core isrelatively small and the amount of Se content per particle wastoo low to be reliably detected. To get a better picture about thesurface capping groups on these PbSe/PbS tetrapods we charac-terized these via Fourier transform infrared (FTIR) spectroscopy,which showed that aside from TOP, the ligands present on theas-synthesized PbSe seeded PbS tetrapods contained oleic acidand n-octadecylphosphonic acid (ESI Fig. S6†).

The construction of the NC-based IR photodetector devicewas carried out by drop-casting under inert conditions a con-centrated solution of PbSe seeded PbS tetrapods in tolueneonto pre-fabricated gold electrodes atop and supported on aglass substrate. The inter-electrode separation was fixed at10 µm and the electrode height was determined to be∼800 nm. An overall schematic of the device architecture andphotoresponse measurement is given in Fig. 3a. It should beemphasized that unlike previous efforts to fabricate PbS NCphotodetectors which typically involved ligand exchange of theoleic acid-capped PbS with shorter alkyl amine ligands19,50

and/or treatment of the PbS NC film with 1,2-ethanedithiol(EDT) to passivate electron traps,24 the PbSe seeded PbS tetra-pods did not undergo any ligand exchange process or filmtreatment. Post-annealing of the film at elevated temperatures,which is a strategy often employed to increase the device per-formance,24 was also not carried out. Fig. 3b and c show thetop and cross-sectional scanning electron microscopy (SEM)images respectively of densely-packed PbSe seeded PbS tetra-pods with a thickness of approximately 3 µm across the centralregion of the dropcast film. No obvious cracks or patchy areason the order of ∼1 µm were observed, suggesting that theoverall morphology of the active area of the film was fairlysmooth and uniform. Atomic Force Microscopy (AFM)

measurements on the resulting tetrapod film, as exemplifiedin Fig. 3d, yielded a smooth morphology with a typical surfaceroughness (RMS) of about ∼6.5 nm, over a 5 μm2 scan area,suggesting a uniform tetrapod film.

In order to evaluate the ability of photoresponse from atetrapod-based device, photocurrent measurements were doneunder vacuum with four different continuous wave (CW) broadbeam laser sources (405 nm, 532 nm, 808 nm and 1064 nm)with a fixed laser intensity of 5.7 × 10−2 W cm−2 in the experi-mental setup configuration as stated in the Experimentalsection.53,54 Broad beam illumination means when the laserirradiation illuminates the full device structure, where theactive sample and electrode contacts get illuminated underlaser light. Upon irradiation with four different laser sources,the PbSe/PbS films clearly show the highest photocurrent at anear infrared laser (808 nm) source, as can be seen in Fig. 4a.The I–V response of the device at various excitation sources, asin Fig. 4a, showed an asymmetry in the current generatedbetween the positive and negative bias applied which may beattributed to structural unevenness in the fabricated Au elec-trodes and the formation of uneven Schottky contacts betweenthe electrodes and the tetrapod sample. Under different broad

Fig. 3 (a) Schematic of the tetrapod-based IR photodetector based ona lateral electrode configuration. (b), (c) are the top and cross-sectionalSEM images respectively of densely-packed PbSe/PbS tetrapod filmsdropcast from a solution of tetrapod particles dispersed in toluene. Thethickness at the central region of the film was about 3 µm. (d) Represen-tative AFM image of a typical PbSe/PbS tetrapod’s film onto a glasssubstrate.

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beam irradiation the I–V responses maintain similar asymme-trical characteristics as the dark I–V. Between an applied biasof −2.0 V and +2.0 V it is observed that there is little photo-current enhancement, (see the inset of Fig. 4a) while at higherapplied bias a dramatic increase in the photocurrent isobserved. The onset of the large increase in photocurrent andto some extent the dark current at a moderate bias of +2.0 V issuggestive of carrier injection from the Au electrodes into thefilm, similar to what was observed in a Bi2S3 nanorod photo-detector with Au contacts.43 This is plausible given that thework function of the Au electrodes is significantly loweredwhen exposed to organic molecules,55 and may be relativelyclose to the conduction band (CB) of the 2-D quantum con-fined PbS tetrapod arms (CB of bulk PbSe is ∼4.6 eV (ref. 56))such that carrier injection occurs under an applied bias. Theinjection of carriers and photogenerated carriers under lightillumination into the film results in an overall increase incurrent and consequently enhanced photoconductive gain.The photocurrent responses (I–t curves) of the PbSe/PbSdevices under multiple on/off sequences are shown in Fig. 4b.The plot shows that the current increased to the ON state uponillumination of the laser beam. When the laser irradiation wasswitched off, the photocurrent returned back to its originalvalue (OFF state). The devices were prompt in generating a

photocurrent with a reproducible response to ON–OFF cycles.Notably, Fig. 4b confirmed the maximum response under illu-mination of 808 nm-light, which makes these tetrapods suit-able for NIR photodetection applications. One of the mostimportant attributes defining the efficiency of a photodetectoris its responsivity (Rλ), which is essentially the amount of netphotocurrent derived for a given area and intensity of photo-excitation at a fixed excitation wavelength. The responsivity ofthe PbSe seeded PbS tetrapod photodetector at 808 nm and anapplied bias of +4.0 V is ∼11.9 A W−1, which is significantlyhigher than the other three different light sources as shown inFig. 4c. The responsivity value at 808 nm is comparable in col-loidal semiconductor nanoparticle based NIR photodetectorswith a single active material. It should be noted that theaddition of a secondary material with high carrier mobilitysuch as graphene can increase the responsivity substantially,as exemplified by recent reports of hybrid PbS QD/graphenebased photodetectors with responsivities as high as ∼107 AW−1.51,52 In this work, however, we have limited our NIRphotodetector device architecture to having the semiconductortetrapods as the only active component in order to investigatetheir merits as NIR photodetectors. A possible explanation forsignificant decrease of responsivity at 405 nm and 532 nmexcitation sources is the filling of charge trap states. These trapstates allow for the achievement of high photoconductive gainby prolonging exciton recombination times and preventingextraction of the trapped carrier,57 and the suppression ofthese trap states would result in a reduced photocurrent.Although photo-annealing at higher excitation intensities canalso facilitate the elimination of trap states by removing crystaldefects,58 such a scenario is unlikely applicable in this casesince control experiments showed that cycling between thedifferent excitation intensities at a fixed bias reproduced thesame photocurrents, suggesting that the non-participation ofthe trap states at high excitation intensities is transient andconsistent with a state-filling process. On the other hand at a1064 nm light source, the excitation energy is lower than theband edge energy of the PbSe/PbS system to induce excitongeneration. In addition, there is also a thermal heating effectfrom an illuminated laser source, which gives rise to free car-riers in the device although the effect might be negligible. It isfound that infrared light irradiation has a higher heating effectin nanocrystals/metal electrode junction rather than thevisible light illumination. This heating effect could generateadditional charge carriers which can contribute as photo-current. This is possible as more energy releases when NIRlight hits semiconductors. A cation exchange process alwayscreates some defects which cause even deep trap states. Thesetrap states can trap the charge carriers and prolong their lifetime which causes a gain in the photodetector device. Now ifthe incident light has sufficient energy, the trapped carrierscould be released and can recombine with their oppositepartner. In our case, it might be the scenario where 532 nmlight could reduce the carrier life time by effectively excitingthem even from trap states. This results a loss in gain andincreasing trend of carrier recombination and both in combi-

Fig. 4 (a) Graph of photocurrent vs. applied voltage at different lightexcitations: 1064 nm (blue), 808 nm (red), 532 (green), 405 (purple). Thedark current curve is shown in black. The inset is a zoomed in the regionof the I–V curve from 0.0 to 2.0 V. (b) The temporal response of thephotodetector at different light excitations: 1064 nm (blue), 808 nm(red), 532 (green), 405 (purple) with fixed laser intensity of 5.7 × 10−2

W cm−2 and +4 V applied voltage. (c) Responsivity of the PbSe seededPbS tetrapod-based photodetector as a function of the excitationsource with a fixed laser intensity of 5.7 × 10−2 W cm−2 and +4 V appliedvoltage. (d) A zoomed in the region of the temporal response curveunder 808 nm light irradiation in (b), where a response time of ∼0.1 swas determined from the photocurrent decay. The rising curve is fittedby the equation I = I0 + A1(1 − e(t0 − t/τ1)), where I0 is the dark current, t0is the initial time, A1 is the amplitude of photocurrent, and τ1 is the risingresponse time which is ∼1 s.

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nation reduce the photocurrent at 405 and 532 nm excitation.The temporal response of the photodetector was evaluated andis depicted in Fig. 4b, where it is readily seen that the photo-current generated between ON/OFF cycles is fairly steady overthe course of the cycle time, which in this case was a little overan hour. The slight fluctuations in the photocurrent may beascribed to the effects of heating, which can result in changesin conductivity across the film.59 Fig. 4d is a magnified regionof Fig. 4b, and the photocurrent decay time is approximately∼0.1 s, whereas the rising time is determined to be ∼1 s. Twodifferent photoresponse times were observed which could bedue to the fact of two uneven Schottky contact formation wherevarious trap states and intermediate gap states are present. Theslow time response is due to the traps, defects (because of a rig-orous cation exchange process) and interface state originatedphoto carriers, which take time to make the full ON state of thedevice. And similarly it takes a long time (more than 0.1 s) toreach the fully OFF state, which also has a slow time constantdue to carrier trapping. The resulting photogenerated currentis due to a combination of effects, in which the interface andtrap states play a major role. These PbSe/PbS nanocrystals deve-loped huge defects during prolong (96 h) cation exchangesynthesis, thus creating charge trapping sites and grain bound-aries with low carrier mobility, which supports our claimbehind a dominated factor of the slow time response. Neverthe-less, such a temporal response is sufficient for most imagingapplications that do not require very fast capture rates.

External quantum efficiency (EQE),60 another importantparameter for photodetectors, is defined as EQE = (hc/eλ) × Rλ,where h is the Planck’s constant, c is the speed of light, e is theelectronic charge, λ is the excitation wavelength and Rλ is theresponsivity at that excitation. The EQE of the device is esti-mated to be ∼1240%, 615%, 1824% and 253% for the excitingwavelengths of 405 nm, 532 nm, 808 nm and 1064 nm, respec-tively. Evidently the maximum value of EQE was obtained at808 nm-light. However EQE considered as the gain is oftenreported more than 100%, which is possible if there is carriermultiplication via multiple excitons in nanocrystals.61 Thishappens when the mean carrier lifetime exceeds the transittime in the device. Carrier lifetime is a strong function of thesemiconductor materials used in the photoconductor. It ispossible for the photoconductor gain to be much greater thanunity if the carrier’s lifetime can be made to significantlyexceed the transit time. This corresponds to a photogeneratedcarrier looping through the circuit many times before recom-bining. Each time a photocarrier makes a complete tripthrough the circuit it contributes a charge of q = 1.6 × 10−19

coulombs to the current flow in the circuit.To shed more light on the underlying mechanism of the

superior photocurrent response of the PbSe/PbS tetrapods, thephotoinduced electron transfer dynamics between PbSe andPbS were investigated using nanosecond transient absorptionspectroscopy (ns-TAS). These techniques allow one to monitorthe electron and hole state filling kinetics in the lowest energylevels and are widely used for investigating the charge carrierdynamics in semiconductor nanostructures.62,63 Aside from an

expected increase in charge percolation pathways due to thebranched morphology of the tetrapods, another possible con-tributing factor to the high photoconductive gain achieved isthe band offsets between PbSe and PbS, which from bulk con-siderations form a Type II energy alignment as illustrated inFig. 5a. Thus excitons generated would spontaneously separ-ate, leaving the electrons in PbSe and the holes in PbS andresulting in significantly lengthened exciton lifetimes. To elu-cidate the long-lived charge carrier recombination lifetime, ns-TAS was performed to monitor the photoinduced absorption(PIA) from the band edge and/or localized states to higherenergy excited states in the visible range. Fig. 5b illustrates theTA spectra from 525 nm to 750 nm, which features a promi-nent PIA peak at ∼680 nm. The PIA dynamics at 680 nm werethus monitored, and the decay curve and multi-exponentialfits are shown in Fig. 5c. A long-lived decay transient with anintensity weighted decay time of ∼17 ms was observed, whichis much longer than the photoluminescence lifetime charac-teristics of PbS and PbSe NCs.64 The fast electron transfer andlong-lived charge separated states in these PbSe/PbS tetrapods,which may arise from a combination of a Type II band align-ment between PbSe and PbS and a large density of surfacestates that were formed during the series of cationic exchange

Fig. 5 (a) Schematic of the band alignment and charge dynamics underlight of the PbSe seeded PbS tetrapods, with respect to their bulkvalues. (b) Differential transmission spectra in the visible range for thetetrapods in toluene at a probe delay of 1 ps following a ∼50 μJ cm−2

pump pulse at 400 nm. (c) PIA kinetics at 680 nm monitored using ns-TAS. The decay transient yielded an intensity weighted decay time of∼17 ms.

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processes, could be possible mechanisms for the acceptableresponsivity and the moderately high photodetection efficiencyin our devices.

Conclusions

In summary, we have demonstrated the synthesis of Cu2−xSe/Cu2−xS tetrapods from CdSe/CdS tetrapods which are used as‘templates’ for cation exchange reactions with excess of copper(I) salt as a precursor. This Cu2−xSe/Cu2−xS nanocrystal wasfound to be imperative as a versatile intermediate material fora sequential cation exchange reaction. In the second cationexchange reaction, PbSe/PbS tetrapods were synthesized byreplacing the Cu+ cations with Pb2+ cations with an excessamount of lead(II) salt precursors and TOP as a complexingagent. The cation exchange synthesis of both Cu2−xSe/Cu2−xSand PbSe/PbS tetrapods was successfully reported with un-precedented shape homogeneity and monodispersity of the nano-crystals and is believed to be highly novel. Next we developed aNIR photodetector device which comprised dropcast PbSe/PbStetrapods as an active material film sandwiched between twocoplanar Au electrodes on a glass substrate. Despite the exclu-sion of laborious ligand exchange processes and thermalannealing to improve the conductivity of the nanoparticle film,the tetrapod-based device was able to achieve a responsivity of∼11.9 A W−1. Following that, we proposed a plausible mechan-ism for the photoresponse by charge recombination dynamicsvia transient absorption spectroscopy, which indicated that thebranched morphology was likely a contributing factor to thephotodetection efficiency obtained. This charge recombinationdynamics revealed an extremely long-lived exciton lifetime of∼17 ms, which is highly advantageous for photoconductivegain and may be attributed to the presence of surface trapstates as well as a Type II band alignment between the PbSecore and its PbS arms. These initial results collectively suggestthat heterostructured PbSe seeded PbS nanotetrapods are avery promising alternative to the more widely adopted spheri-cal PbS nanocrystals as the active material in a NC-based NIRphotodetector. We envision that further optimization andhybrid device architecture could lead to improvement in thephotodetection efficiency for possible practical applications.Moreover the new Cu2−xSe/Cu2−xS tetrapods could be useful asa potential material for the plasmonic study as well as appli-cations in electronic and photovoltaic devices. More impor-tantly, the use of cationic exchange processes to deriveanisotropic semiconductor nanoheterostructures can result ina fortuitous combination of properties which may be highlydesirable for solution-processed optoelectronic applications.

ExperimentalMaterials

1-Octadecene (ODE, 90%), oleic acid (OA, 90%), tetrakis(aceto-nitrile)copper(I) hexafluorophosphate (97%), lead(II) acetate tri-

hydrate (99.999% trace metal basis), methanol anhydrous(99.8%) and toluene anhydrous (99.8%) were purchased fromSigma Aldrich. Trioctylphosphine (TOP, 97%) was purchasedfrom Alfa Aesar. All the chemicals were used as receivedwithout further purification. Unless stated otherwise, all thereactions were conducted in oven-dried glassware under anitrogen atmosphere using standard Schlenk techniques.

Synthesis of Cu2−xSe/Cu2−xS and PbSe/PbS tetrapods

The synthesis of PbSe seeded PbS tetrapods is carried out byperforming sequential cationic exchange reactions with Cu+

and then Pb2+ using CdSe seeded CdS tetrapods as the start-ing material following a previously reported cation exchangeprocess.28 The synthesis of CdSe seeded CdS tetrapods wascarried out via a previously established procedure9,10 and pro-cessed in a growth solution using 2–3 cycles of precipita-tion/re-dispersion in methanol/toluene. For the cationicexchange with a Cu(I) reaction to form Cu2−xSe/Cu2−xS tetra-pods, 1 mL of a 100 µM solution of tetrakis(acetonitrile)copper(I) hexafluorophosphate in MeOH was added to a 1 mLsolution of 10 µM of CdSe/CdS tetrapods in toluene. The reac-tion was allowed to stir for ∼10 min before precipitating withexcess methanol, followed by centrifugation at 3900 rpm for∼20 min. The resulting Cu2−xSe/Cu2−xS tetrapods were thenredispersed in 1 mL of toluene for subsequent cationexchange with Pb2+. In order to carry out this reaction, the1 mL Cu2−xSe/Cu2−xS tetrapods in toluene was mixed with∼20 times molar excess of lead(II) acetate trihydrate dissolvedin 1 mL methanol. Separately, another mixture containing0.25 g lead(II) acetate trihydrate, 0.5 mL oleic acid and1.22 mL of 1-octadecene was heated to 250 °C under N2 for∼5 min and added to the mixture of Cu2−xSe/Cu2−xS tetrapodscontaining the excess lead(II) salt. To this overall mixture,100 μL of trioctylphosphine was added, and the reaction wasthen allowed to proceed with vigorous stirring for 96 hoursinside a glove box at room temperature, which resulted inPbSe/PbS tetrapods.

Structural characterization

Transmission electron microscopy (TEM). A JEOL JEM1220F (100 kV accelerating voltage) or JEOL 2100 (200 kV accel-erating voltage) microscope was used to obtain TEM images ofthe nanoparticles respectively. For TEM measurements, a dropof the nanoparticle solution was placed onto a 300 mesh sizecopper grid/nickel grid covered with a continuous carbon film.Excess solution was removed by using an adsorbent paper andthe sample was dried at room temperature. The elementalcomposition analyses were carried out using a FEI Titan80–300 electron microscope operated at 300 kV, which isequipped with an electron beam monochromator, an energydispersive X-ray (EDX) spectroscope and a Gatan electronenergy loss spectrometer. The probing electron beam size forthe EDX measurement was around 0.3 nm, with a dwell timeof ∼10 s for each EDX spectrum.

Scanning electron microscopy (SEM). To measure the thick-ness of the dropcast NC film on the prefabricated electrodes,

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we dropcast a similar amount of a concentrated NC solutionon a Si substrate and evaporated the solvent at 80 °C. We usedthis dropcast Si substrate to examine the cross-sectional thick-ness using field emission scanning electron microscopy(FESEM, JEOL JSM-6700F).

Atomic force microscopy (AFM). Surface roughnesses weremeasured by a Bruker Dimension FastScan AFM machine oper-ating in tapping mode (FASTSCAN-A). “Nanoscope Analysis”software is used to determine the height profile of the nano-particle-like structure based thin film by making a scratch onthe continuous film, which is deposited onto the glasssubstrates.

XRD characterization. X-ray Diffraction (XRD) data wereobtained with a diffractometer (Bruker AXS, GADDS) using Cu-Kα radiation (λ = 1.54 Å) in the range of 20° to 80°. Sampleswere prepared on a clean silicon wafer by placing drops of con-centrated nanoparticles in toluene on the silicon surface anddried at room temperature inside a glove box. This wasrepeated several times until a thin layer of solid was formed onthe silicon substrate.

Optical characterization. Absorption spectra were recordedwith a Shimadzu UV-3600 and UV-Visible-NIR spectrophoto-meter. Care was taken to ensure that the concentrations ofthe core and core-seeded nanostructures were sufficientlydilute to avoid contributions from re-absorption or energytransfer.

FT-IR measurement. The FT-IR data were recorded on aFTS165 Bio-Rad FTIR spectrometer, where toluene and ahighly concentrated QD solution were placed between theNaCl plates and checked the FT-IR spectra in transmissionmode.

Device fabrication. Au bar electrodes were fabricated on aglass substrate via e-beam evaporation and a liftoff process fol-lowing previously reported methods.49,50 The gap between twoAu electrodes was 10 μm. The as-synthesized PbSe seeded PbStetrapods were precipitated from the solution with methanoland re-dissolved into toluene at a concentration of 75mg mL−1. A drop of NCs deposited on the prefabricated Auelectrodes in this manner resulted in a total film thickness of3 μm as determined by field emission scanning electronmicroscopy (FESEM, JEOL JSM-6700F).

Device measurements. Current–voltage characteristics weremeasured using a Keithley-6430 source-measurement unitboth in the dark and under light illumination.53,54 All opto-electrical measurements of the NC-based film were conductedwhen the devices were placed inside a vacuum chamber (10−3

mbar) and the sample was illuminated through a transparentglass window of the chamber. The electrical wires from thedevice were fed through a pair of vacuum compatible leads tothe measurement unit. A diode laser at 808 nm (EOIN) wasused to illuminate and obtain photoresponse from the device.To measure the time response, a LQD-1060 laser diode, modu-lated at 5 Hz, was used to illuminate the devices and a FemtoDHPCA-100 High Speed Current Amplifier and Tektronix3054b oscilloscope was used to measure the temporal photo-current decay.

Optical setup. For nanosecond transient absorption spec-troscopy (ns-TAS), a laser flash photolysis spectrometer(LKS.60, Applied Photophysics), equipped with a Q-SwitchedNd:YAG laser (Brilliant B, Quantel), a 150 W pulsed Xe lampand a R928 photomultiplier, was used to record ns-differentialabsorption spectra. The samples were excited at 400 nm andeach time-resolved trace was acquired by averaging 10 lasershots at a repetition rate of 1 Hz.

Acknowledgements

We gratefully acknowledge funding support from JSPS-NUSJoint Research Projects WBS R143-000-611-133 and financialsupport by the National Basic Research Program of China-Fun-damental Studies of Perovskite Solar Cells (2015CB932200).We would like to thank Peter Schulze & Dr Kiran KumarManga for fruitful discussion.

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