and Composition-Controlled Thallium Lead Halide Perovskite

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Shape-, Size- and Composition-Controlled Thallium Lead Halide Perovskite Nanowires and Nanocrystals with Tuneable Band Gaps Parth Vashishtha, Dani Z. Metin, Matthew E. Cryer, Kai Chen, Justin M Hodgkiss, Nicola Gaston, and Jonathan E. Halpert Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00421 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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Chemistry of Materials

Shape-, Size- and Composition-Controlled Thallium Lead Halide Perovskite Nanowires and Nanocrystals with Tuneable Band Gaps Parth Vashishtha†, Dani Z. Metin‡, Matthew E. Cryer†, Kai Chen†, Justin M. Hodgkiss†, Nicola Gaston‡, Jonathan E. Halpert†§* †

MacDiarmid Institute for Advanced Materials and Nanotechnology, and School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand ‡ MacDiarmid Institute for Advanced Materials and Nanotechnology, and Department of Physics, University of Auckland, Private Bag 92019, Auckland, New Zealand § Department of Chemistry, Hong Kong University of Science and Technology (HKUST), Kowloon, Hong Kong

ABSTRACT: Perovskite nanocrystals have shown themselves to be useful for both absorption and emission-based applications, including solar cells, photodetectors and LEDs. Here we present a new class of size, composition- and shape- tunable nanocrystals made from Tl3PbX5 (X= Cl, Br, I). These can be synthesized via colloidal methods to produce faceted spheroidal nanocrystals, and perovskite TlPbI3 nanowires. Crystal structures for the orthorhombic and tetragonal phase materials, for both pure and mixed halide species, are compared to literature and also calculated from first-principles in VASP. We show the ability to tune the band-gap by halide substitution to create materials that can absorb strongly between 250– 450 nm. In addition, we show evidence of the confinement effect in pure halide Tl3PbBr5 nanocrystals suggesting size-tuning is possible as well. By tuning the band-gap we can create materials with specific absorption spectra suitable for photo-detection that display conduction and photoresponse properties similar to previously observed perovskite nanocrystals. We also observe weak emission consistent with indirect band-gap materials. Finally, we are able to demonstrate shape control in these materials, to give some insight into observable phase changes with varying reaction conditions, and to demonstrate the utility of the TlPbI3 perovskite nanowires as wide-band gap photoconductors. These novel perovskite nanocrystalline materials can be expected to find applications in photodetectors, X-ray detectors, and piezoelectrics. Recently perovskite nanocrystals (peNCs) have demonstrated an array of novel physical and optical properties for quantum dots.1-6 In addition to the quantum dot properties of size tuneable band gaps, MAPbX3 (MA = methylammonium) and CsPbX3 nanocrystals (NCs) have demonstrated unusual properties such as high quantum yield of emission,1,7 facile inter-particle anion exchange,8 relatively high charge mobility and conductivity,9-10 and low temperature particle fusion to form 1-D and 2-D structures.11-14 These observations have driven a number of studies into alternative NC structures based upon the perovskite model, including FAPbX3, MA3BiX9, Cs3BiX9, CsSnX3, Cs3Sb2Br9, (where X = Cl, Br, I, FA = formamidinium) as well as other perovskite analogues.15-23 These can be said to include double perovskites, alloys, mixed halide, doped perovskites and other nanocrystalline materials analogous in some way to methylammonium lead iodide (MAPI).24-27 MAPI, of course is the well characterized 3-D structured perovskite used in high efficiency thin film solar cells and LEDs.28-33 Finding novel perovskites and related materials with useful, similar properties is currently a major area of nanomaterials research and many groups are working to replace the A, B, and X groups in the ABX3 structure with other feasible atoms.17-18,22 One such candidate for replacing methylammonium as the A group, in the ABX3 perovskite crystal structure, is thallium. Thallium is an abundant monovalent cation with a crystalline radius of roughly 1.61 Å. It is known best for its toxicity, including carcinogenic and teratogenic effects, and has only once been used in a reported quantum dot, as a binary halide salt.34,35-36 Monovalent Tl+ readily forms in-

soluble perovskites and analogous crystals in combination with a range of lead halides.37-39 This family of bulk crystalline materials are generally found to be high energy (2.5–4.0 eV), indirect band-gap materials with several known applications, such as X-ray and UV-vis photodetectors, IR photodetectors when doped appropriately, and as piezoelectric materials.38,40-44 Thallium lead triiodide (TlPbI3), for example, is a perovskite type material structurally similar to CsPbBr3 and a wide bandgap semiconducting material that has been predicted to have piezoelectric properties.38 It could be a candidate for X-ray detection due to its high atomic number (Z), wide bandgap, and high density (6.6 gm∙cm–3). It is known to have a higher absorption cross section than CdTe and other commonly reported QD materials.42,44 Having a higher bandgap (Eg > 1.4 eV) also prevents the thermal generation of charge carriers, improving signal to noise at room temperature.42,44 Other thallium lead halide species, Tl3PbBr5 and Tl3PbCl5 have been used as mid-infrared (MIR) nonlinear optics and for near-IR non-linear optical effects.38,40,43 For example, rare-earth doped Tl3PbBr5 is considered a promising material for mid-IR solid-state lasers; quantum dots of this material could be an even more attractive material for that application.41,45-46 Here we present a facile synthesis of thallium lead halide nanocrystals and nanowires as novel analogues to other nanocrystalline inorganic lead halide pervoskites.1,20-21,23 Orthorhombic nanocrystals with the chemical formula Tl3PbX5 [X=I, Br] and tetragonal nanocrystals of Tl3PbCl5

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were synthesized with a wide range of direct transition

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peaks across the

Figure 1: High resolution transmission electron micrographs (HR-TEMs) for a) ~17.2 nm, b) ~28.0, and c) ~20.5 nm NCs, X-ray diffraction spectra of these same NCs, matched to index spectra (X’pert high score), as well as spectra theoretically modelled in VESTA, and their respective periodic crystal structures,47-48 which have been optimized in VASP, for a) Tl3PbI5, b) Tl3PbBr5, and c) Tl3PbCl5 UV-blue spectrum, from 440 nm to 280 nm (Figure 3). Band-edge tuning was demonstrated both by tuning the halide mixture in mixed-halide Tl3PbX5-xBrx NCs [X=I, Cl], and, to a lesser degree, by size tuning. A size series of Tl3PbBr5 nanocrystals were used to demonstrate the confinement effect in these materials, and weak photoluminescence is observed in Tl3PbI5 NCs. Furthermore, we demonstrate control over the shape and crystal structure of thallium lead iodide by altering the reaction conditions to produce TlPbX3 perovskite nanowires with high aspect ratios and lengths greater than 4 µm. We find that these materials are air-stable for several weeks. Results and Discussion: Synthesis. Thallium lead halide NCs were synthesized in a two-step reaction in which thallium acetate was heated with oleic acid to produce thallium oleate, which was then stored under N2. In the second step, Tl-oleate was injected

into a heated pot under N2, containing pure or mixed lead halides PbX2 (X = I, I/Br, Br, Cl/Br, Cl) in oleylamine, oleic acid and 1-octadecene to form nanocrystals. The NCs were grown for 30-45 seconds at high temperature before cooling to room temperature. They were then purified by between two and four solvent/anti-solvent cycles before being finally re-dispersed in hexane. The results of several variations on this procedure are summarized in Table S1, to produce both pure and mixed halides as well as perovskite structured nanowires from the iodide precursor. Crystal structure. The NCs produced by this method, shown in Figure 1, appear as faceted spheroidal particles, with XRDs matching those of the bulk orthorhombic phases of Tl3PbI5 and Tl3PbBr5 and the tetragonal phase of Tl3PbCl5, respectively. NCs of all species can be seen to be highly crystalline in high resolution transmission electron microscope (HR-TEM) images, with visible lattice fringes


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matched to the (312), (021) and (213) crystal planes (Figure S4) of the lattice structures with symmetry P212121, P212121, and P41, respectively. XRD peaks for NCs are broadened compared to the bulk index peaks according to the Scherrer equation.

Crystallographic structures of the tetragonal Tl3PbBr5 and the

Figure 2: Histogram with the TEM micrograph of a) Tl3Pb(I0.43/Br0.57)5 NCs synthesized at 140 ˚C. NCs have 15.2 nm diameter with 2.2 nm standard deviation. Particles were dissolved in hexane before the TEM sample preparation; c) Tl3Pb(Cl0.36/ Br 0.64)5 NCs synthesized at 130 ˚C. NCs have 19.2 nm diameter with 1.7 nm standard deviation. Particles were dissolved in hexane before the TEM sample preparation; b) and d) X ray diffraction spectra of the same NCs, matched to the theoretically modelled crystal structure with two different halogen arrangements (see S9 and S10).

orthorhombic Tl3PbBr5 and Tl3PbI5 are based on the work of Keller48,47 but are fully optimized using first principles calculations. The Goldschmidt tolerance factors for each perovskite were calculated to be 0.772, 0.778, 0.780 for I, Br, and Cl, respectively, and are all in the range of stable orthorhombic perovskite structures.49 Despite this, bulk ABX3 perovskites of thallium lead bromide and chloride have not been reported at atmospheric pressures.50 In this work, low dimensional NCs all formed the preferred reduced symmetry structure, A3BX5, while TlPbI3 nanowires (NWs) were able to be synthesized at higher temperature over longer reaction times. The agreement between XRD calculated from the periodic 3D lattice and the experimental XRD pattern suggests that this is indeed the correct structure. Mixed halides, shown in Figure 2, display intermediate XRDs with some peaks shifted slightly between those of the two pure materials (see S9 and S10 for detailed discussion). Tl3Pb(Br/I)5 and Tl3Pb(Br/Cl)5 mixed halide XRD spectra were fitted to simulated spectra produced in VESTA57 using structures descended from the bulk low-temperature orthorhombic crystal structure of

Tl3PbBr5,47 which has been optimized using the Vienna AbInitio Simulation Package (VASP)51-53 with two variations in halides position positions (Figure S9.2 and S10.2). Both structures fit reasonably well, suggesting that the distribution of Br and I is not well-ordered since higher order peaks could not be observed. Optical Properties. Absorption spectra are shown in Figure 3. The pure halide Tl3PbX5 NCs show evidence of both a strong first direct transition peak, typical of semiconductor nanocrystals under weak and intermediate confinement,54 where the width of the peak is related to the size distribution. In addition to the first transition peak, a weak, lower energy indirect transition is also observed. This is in agreement with theoretical and experimental observations of bulk Tl3PbI5, Tl3PbBr5 and Tl3PbCl5.38-39,55 The energy of the band gaps, direct and indirect, increase in energy as you go up the halide series from I to Cl, as would be expected for semiconducting halides with similar crystal structures. Tauc plots are shown in Figure S5, with extrap-



olated band edges for both direct and indirect band edge transitions peaks as estimates of the band gap energy. Mixed halide NCs displayed larger full-width half maxima (fwhm) for their first strong transition peak. The direct band gap energy of these materials were found to be intermediate value between their pure halide parent NCs, allowing tuning by halide composition across the visible blue to UV regions as shown in Figure 3(c). This data is

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summarized in Table S2, along with the peak wavelength and estimated fwhm of the first optical transition as determined by a simple Gaussian fit (Figure 3, dashed line). The first strong absorption peaks were found to range from 281 nm to 440 nm, indicating the

Figure 3: a) Absorption spectra of TlPbI3 nanowires and Tl3PbI5, Tl3PbBr5, and Tl3PbCl5 nanocrystals along with mixed halides I/Br and Cl/Br synthesized via a 1:1 reaction mixture, with a resulting 57% and 64 % Br content, respectively. b) Emission spectra observed at high pump excitation density for Tl3PbI5 NCs display the wide emission across the visible spectrum. c) A chart of the effect of composition on the direct band gap, as measured by the first strong absorption peak.

feasibility of using these materials for detecting deep blue to moderate UV photons. Emission from Tl3PbI5. Weak emission was observed in Tl3PbI5 NCs excited at high pump excitation density (~ 42 µJ/cm2) and is shown in Figure 3(b), along with the PL lifetimes in Figure S2. Emission could not be observed in other samples (see Methods section). The broad emission peak across the white light spectrum from 450-600 nm does not fit the profile of trap emission in NCs but rather can be decomposed into two Gaussian curves (Figure 3(b)). There is additionally some small intensity observable beyond 650 nm, which is indicative of trap emission.

The presence of a visible shoulder to the curve and the poor fit of a single Gaussian profile, further justifies this approach.56-58 The first fitted peak at 554 nm (2.24 eV) accounts for 55% of the integrated emission signal. This value agrees with the literature indirect band gap energy of 2.29 eV for emission in bulk Tl3PbI5 at 300 K.38 It is notable that direct/indirect emission co-contributions have been recently reported in other similar perovskite analogues.58 However, due to the discrepancy in the long decay lifetimes (τ2), the fitted peak at 498 nm is more likely to arise from high energy trap sites, from smaller NCs with blue-shifted band gaps, or from recombination from the 1st


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strong (direct) transition observed in the absorbance spectrum. Neither peak matches the reported emission from binary thallium halide salt NCs, which was reported to occur at ~2.7 eV.36 The PL peaks were measured using time-resolved photoluminescence and each peak was fitted to a double exponential decay (Figure S2). These decays showed sub-

nanosecond decay pathways, shorter than the instrumental limit (< 1 ns), and long decay pathways of 4.4 ns and 2.7 ns for the 498 nm and 554 nm PL peaks, respectively. The large percentage of the signal intensity decaying via the short pathways (τ1) for both signals, 72 % and 90 %, respectively, suggests the material has a dominant, and fast non-radiative decay path. Given the large band-gap energy, low quantum yield and the lack of Type-I shell to

Figure 4: (a, b, c) HRTEM micrographs showing three sizes of Tl3PbBr5 NCs. d) Absorption spectra for the 3 samples, with mean diameters of 28.0 nm, 20.9 nm and 16.9 nm. 1st absorption peaks were identified using a Gaussian curve fit to determine the change in the direct band gap with size. e) A plot of the direct band gap (as estimated from the 1st strong absorption peak) vs. the nanocrystal radius, and fitted to the Brus Equation (Eq. 1).

passivate the surface, the presence of inter-band trap and surface trap states is likely. Additionally, the presence of multiple trap sites would be expected to assist in nonradiative decay processes, especially for an indirect band gap emitter. The difference in decay rates for these peaks does indicate that two different processes are occurring. However, it is apparent that the emission mechanism from these materials is complex and a detailed spectroscopic investigation may be needed to better assign each peak.

Confinement in Tl3PbBr5. Similar to other recent perovskite analogues, a weak confinement effect was observed in Tl3PbBr5 NCs as demonstrated in Figure 4, with a size series of Tl3PbBr5 NCs with diameter 28.0 ± 2.5 nm, 20.9 ± 3.0 nm and 16.5 ± 2.5 nm for samples NC28.0, NC20.9 and NC16.5 respectively. These are shown in the histograms overlaid on the HRTEM images in Figure 4(a-c). Given the lack of observable emission, the first strong transition peak was used to measure the shift in the direct band energy with size and was estimated by fitting the first absorption



peak energy. When plotted as an estimate of the direct band gap (first allowed transition), these peaks could be fitted to the Brus equation for spherical particles.59-60

E = Eg +

h 2π 2 µ ex* 2r 2

 1 1  + * * m m h   e

−

1.8e 2 4πε r ε 0 r ,

and

(1)

µ* =  ex

(2)

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Where µ*ex is the reduced mass of the exciton, r is the radius, Eg is the bulk band gap, and εr is the relative permittivity. The fitting parameters include the bulk band gap and the reduced mass of the exciton which were permitted to vary, while the relative permittivity (εR,bulk ~ 30) was initially fixed. The bulk direct band gap was found to be 3.65 eV, which agrees reasonably with the reported literature values,39 and the reduced mass of the exciton was found to be 0.0242 m0. Given the assumption for εR, this would imply a binding energy of 57 meV and a Bohr

Figure 5: a) TEM image of TlPbI3 NWs, b) SEM image of TlPbI3 NWs, c) SEM image of the NWs for EDS mapping, and d), e), f) SEM maps for Tl, Pb and I. An ICP-MS analysis confirms the presence of Tl and Pb in a 1:1 ratio (Table S3).


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Figure 6: a) A high resolution TEM micrograph of TlPbI3 nanowires showing the lattice spacing and FFT planes, b) XRD analysis of NWs and comparison with the standard of orthorhombic TlPbI3 crystal structure, and c) the orthorhombic crystal structure of TlPbI3.

Figure 7: Electrical measurements on TlPbI3 NWs including a) a contour map of the conductivity in (S∙m–1) vs. temperature (K) and voltage (V), and b) a contour map showing the responsivity (A∙W–1) versus temperature (K) and voltage (V). radius of 5.2 nm for the first direct transition (2nd excitonic state). These parameters are similar to those reported for CsPbX3 perovskite NCs, albeit here the first direct transition is not the band edge transition.1 Nanowire Synthesis. By changing the reaction temperature and the ligand concentration, thallium lead iodide nanowires were produced using a similar reaction procedure (Table S1). At room temperature, these structures

consist of lead iodide octahedra with a tilt of 32.3° encapsulating the Tl+ ion (Figure 6). Structural parameters were modelled in VESTA using orthorhombic TlPbI3 in the bulk phase from the X’pert highscore software data file (ref. 04015-4526). This match is confirmed by comparison to the nanowire XRD shown in Figure 6(b). This orthorhombic structure has the Cmcm space group with lattice parameters 4.6 Å, 14.8 Å, and 11.8 Å. In a typical sample (Figure 5) the mean width was found to be 45.1 ± 9.3 nm (Figure



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S1.2) with the wires growing along the (133) plane and minimal formation of side products. EDS mapping shows the presence of all elements and ICP-MS analysis confirms the presence of Tl and Pb in a 1:1 stoichiometric ratio.

als show excellent conduction properties with strong absorption and photoconduction in the UV-blue region. Although thallium lead halides only emit weakly, they have potential uses in several optoelectronic applications.

Electrical measurements on Nanowires. Both twoterminal and three-terminal electrical measurements were conducted on the nanowire samples. The NC solution was spin-coated onto the substrate without any ligand exchange and washed with ethanol. Without further treatment, current flow on the scale of nanoamps could be observed. Figure 7(a) shows the two-terminal conductivity of the film when illuminated with a 405 nm laser diode and how it varies with temperature and bias. With no illumination, the current was not measurable, particularly at low temperatures. The behavior here was remarkably ohmic, which can be attributed to the fact that the majority of the conduction between terminals occurs within the nanowires themselves, with some minimal number of hopping events between NWs required to complete the circuit. At a field dependence beyond 5 V bias the behavior becomes remarkably linear with field. Field dependence is a characteristic of hopping conduction, so beyond 5 V there is evidence of another limiting factor than just the hopping rate. The temperature dependence shows that the NWs behave like an intrinsic semiconductor, with some minima of conduction at 150 K, where the balance point between thermally activated carriers and phononic scattering occurs.61

To our knowledge, this is the first use of thallium as the monovalent cation in a perovskite nanostructure. A-group replacements within the ABX3 structure are rare among the metal halide perovskites, and have thus far been limited to methylammonium, formamidinium and cesium, with few exceptions. Here we suggest the possibility of a new family of thallium-based perovskite nanostructures with interesting and novel properties.

As Figure 7(a) implies, the NWs are intrinsic semiconductors and behave as such when exposed to above bandgap light. The dark current at low temperature was below the noise of the measurement (< 500 fA) and at room temperature was in the nA range. Figure 7(b) shows how the photo-conductive responsivity of the device varies with bias and temperature. This confirms the intrinsic behavior of the NWs, as expected the responsivity shows very similar behavior to the conductivity with the slight variation explained by the increasing dark current with temperature. Although the values are low, this is representative of the ligand mediated film conduction rather than any limitation of the NWs themselves. Indeed, the sensitivity and time response of even this insulating film to intensity changes (shown in S9) suggest that the NWs are worthy of future investigation. Three terminal measurements conclusively showed no gating behavior between Vgs = -20 V and +20 V. It would be expected that gating would produce some mobility change in the carriers within the NWs. The lack of field response suggests that the inter-particle spacing remains a major inhibitor to film conduction and thus here the net effect due to changes in carrier mobility is obscured. Conclusions: In conclusion we have synthesized halide substituted orthorhombic Tl3PbXxY5-x with [X/Y= I, Br, Cl] nanocrystals as analogues to other I-IV-VII perovskite-NCs. These materials have size and composition tuneable band gaps, and low estimated excitonic binding energies, which could make them feasible for photodetector and other wide band photovoltaic applications. We have also synthesized thallium lead iodide in a TlPbI3 perovskite structure. These materi-


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Methods: Materials. Oleic acid (90%), thallium(I) acetate (99%), 1octadecene 90%, PbBr2 (99.99% trace metal basis), PbCl2 (99.99% trace metal basis), PbI2 (99 %), oleylamine (99%) were purchased from Sigma-Aldrich. The synthesis process was done in two steps: 1) the synthesis of precursor thallium oleate and 2) the synthesis of nanoparticles and nanowires. Synthesis of Precursor. 0.46 g thallium acetate, 15 ml 1octadecene and 1.6 mL oleic acid are loaded in 50 mL 3neck flask. The reaction mixture was degassed under vacuum at 120 ˚C for 1 h and then heated at 150 ˚C for next 30 minutes under N2. The Tl-oleate solution is transferred in to a Schlenk tube and kept in the N2 glove box. Synthesis of Tl3PbX5 Nanocrystals. 0.38 mmol of PbX2 are loaded in to a 50 mL 3-neck flask along with 10 mL of 1-octadecene. The reaction solution has been degassed under vacuum at 120 ˚C for 1 h. simultaneously, 1 mL of oleylamine and oleic acid was degassed under vacuum in a vial. After degassing, oleylamine and oleic acid were injected into the reaction flask under N2 and the temperature was raised to 130–175 ˚C. After this, 1.1 mL of thallium oleate was quickly injected in to the reaction flask and in 30 seconds it was cooled down using an ice bath. The growth solution was centrifuged at 10,000 RPM for 10 minutes, and then the supernatant was discarded. The precipitate was re-dispersed in 6 mL of toluene with 3 mL of acetonitrile as an anti-solvent and centrifuged again at 10,000 RPM for 10 minutes. After discarding the supernatant, the precipitate was dried under vacuum for 5 minutes and then re-dispersed in hexane. This purification process could be repeated for at least two cycles in all applications and as many as four cycles for elemental analysis. Synthesis and Purification of TlPbI3 Nanowires. 0.38 mmol of PbX2 were loaded in to a 50 mL 3-neck flask along with 10 mL of 1-octadecene. The reaction solution was degassed under vacuum at 120 ˚C for 1 h. Simultaneously, 0.5 mL of oleylamine and oleic acid was degassed under vacuum in a vial. After degassing, oleylamine and oleic acid were injected into the reaction flask under N2 and the temperature was raised to 170 ˚C for hot injection. 0.9 mL of thallium oleate was quickly injected in to the reaction flask and in 30 seconds, it was cooled down with the ice bath. The growth solution was centrifuged at 10,000 RPM for 10 minutes. After that, the supernatant was discarded and the precipitate was re-dispersed in 6 mL of toluene (solvent) with 3 mL of acetonitrile (anti-solvent) and centrifuged again at 10,000 RPM for 10 minutes. After discarding the supernatant, the precipitate was dried under vacuum for 5 minutes and then re-dispersed in 10 mL hexane. Theoretical Calculations. For first-principles calculations, Vienna Ab-Initio Simulation Package (VASP) was employed, with the Projector Augmented Wave (PAW) method.62-63 For computational tractability, all structures were optimized at the PBEsol level,64 allowing unit cell volume to relax. A k-grid of 4 × 8 × 8 was used. A criterion of 10−4

eV was required for stopping the electronic convergence loop, and the tetrahedron method was employed. For structural visualization and XRD prediction, VESTA was used with a wavelength setting of 1.5418 nm.65 Starting structures were derived from experimentally-known structural parameters of the bulk orthorhombic Tl3PbBr5 and tetragonal Tl3PbCl5.47-48 The theoretically calculated thallium XRD patterns were compared to experimentally obtained XRD patterns for the nanocrystals. Electrical Measurements and Devices. The devices used for electrical measurements were Si/SiO2 wafers with 300 nm oxide layers. The first step in fabrication was surface treatment ((3-aminopropyl) trimethoxysilane in dry toluene for 30 minutes) to deposit a silane self-assembled monolayer (SAM). Cr/Au (5/50 nm) inter-digitated electrode (IDE) (34 fingers per side, 2 mm long with 20 μm spacing, device area is 5×10-6 m2) networks were deposited by a 1-step process using photolithography. The NWs were spin-cast from hexane onto the IDEs at 1000 RPM for 40 s. After each layer, the NWs were quickly rinsed in ethanol and then dried with nitrogen. Before measurement the base and edges were cleaned with chloroform. Electrical measurements were conducted on a Keithley SCS-4200 parameter analyzer. The device was placed on a temperature controlled mount in a Janis VNF-100 liquid nitrogen cryostat and connected to the SCS-4200. Measurements were then conducted in the dark and when illuminated with a Thorlabs 405 nm laser diode (6.4 mw / cm2), the optical path consisted of 5 cm in air and 2 Al2O3 windows (85 % transmission at 405 nm).

ASSOCIATED CONTENT SUPPORTING INFORMATION Supporting information is available free of charge via the Internet at http://pubs.acs.org Histograms and TEM micrographs, carrier lifetime of Tl3PbI5 NCs, synthesis parameters, Crystallography of NCs, Tauc plots, ICP-MS, electrical measurements, crystal structure modelling of NCs

AUTHOR INFORMATION Corresponding Author * Jonathan E. Halpert

Author Contributions Nanocrystals and nanowires were synthesized and characterized by PV. Theoretical calculations were performed by DZM. Electrical measurements were performed by MEC. Timeresolved spectroscopic measurements were performed by KC. All authors have contributed analysis for figures. PV and JEH prepared the initial figures and manuscript. All authors have edited the final text and given approval to the final version of the manuscript.

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Funding Sources J.E.H. and P.V. acknowledge the Royal Society of New Zealand for funding under the Marsden Fund via Grant E2646/3416. J.E.H. also acknowledges funding from the Rutherford Discovery Fellowship via Grant E2675/2990.

ACKNOWLEDGMENT JEH and PV acknowledge the MacDiarmid Institute for Advanced Materials and Nanotechnology for support. We would like to thank Dr. Anna Henning for the valuable discussion in the crystallographic analysis, Dr. Lukas Hammerschmidt for valuable discussion on theoretical chemistry, and Dr. Bruce Charlier for ICPMS analysis. The authors wish to acknowledge the contribution of NeSI high-performance computing facilities to the results of this research. NZ's national facilities are provided by the NZ eScience Infrastructure and funded jointly by NeSI's collaborator institutions and through the Ministry of Business, Innovation & Employment's Research Infrastructure Programme. URL https://www.nesi.org.nz.

ABBREVIATIONS NCs, Nanocrystals; NWs, Nanowires; ICP-MS, inductively coupled plasma mass spectrometry; MA, methylammonium; FA, formamidinium.

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