Iron Selenide

Subscriber access provided by University of South Dakota

Article

Dual-Functional Optoelectronic and Magnetic Pyrite/Iron Selenide Core/Shell Nanocrystals Andrew J. McGrath, Chenlong Yu, Holger Fiedler, Justinas Butkus, Justin M Hodgkiss, and Jonathan E. Halpert J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02643 • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on April 1, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Dual-Functional Optoelectronic and Magnetic Pyrite/Iron Selenide Core/Shell Nanocrystals Andrew J. McGrath,†,‡ Chenlong Yu,† Holger Fiedler,† Justinas Butkus,† Justin M. Hodgkiss† and Jonathan E. Halpert†* †

School of Chemical and Physical Sciences and the MacDiarmid Institute for Advanced

Materials and Nanotechnology, Victoria University of Wellington, Wellington 6012, New Zealand. AUTHOR INFORMATION Corresponding Author *[email protected]

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 27

ABSTRACT: The instability of pyrite nanocrystals’ (NCs) surfaces toward defect formation has until now impeded most applications of these materials. Here, we synthesize FeS2/FeSex core/shell NCs, with a shell largely composed of hexagonal-phase FeSe. The direct band gap of pyrite was not significantly changed due to the iron selenide coating while reduced trap state influence on charge carriers was revealed after the reaction forming the FeSex layer. The core/shell NCs had significantly higher surface electrical resistivity compared to pyrite. In addition the iron selenide shell changed the net magnetic character of the NCs from diamagnetic to ferromagnetic. The core/shell NC synthesis developed here eliminates the issue of intrinsic surface defects in pyrite NCs, and the NCs are expected to find utility as bi-functional NCs in room-temperature

magnetoelectronic

devices

or

in

biomedical

applications.


2

Page 3 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction In recent years, significant progress has been made in the design of next-generation nanocrystalline devices composed of earth-abundant materials.1,2 Iron pyrite (FeS2) in nanocrystalline form is well-known for its highly favourable optical properties, including a lowenergy band gap,3 high absorption co-efficient4 and good carrier mobility.5 These features, combined with low environmental toxicity and extremely low cost, make pyrite an ideal candidate material for NC-based devices.5,6 A large amount of recent work has been dedicated toward engineering pyrite NCs in the solution phase,7–15 and pyrite NCs as light absorbers in photovoltaic (PV) devices in particular are widely reported.6,16,17 However, pyrite is inherently limited as a nanomaterial, due to the strong tendency of pyrite to form defects at the surface, which have a profound effect on its optoelectronic behaviour.18,19 These defects are commonly associated with sulfur-deficient surface states. Due to the high surface-to-volume ratio of NCs compared to the same material in the bulk form, the defective surface of pyrite is an important outstanding issue. Previous approaches toward reducing pyrite’s surface defects include processing the NCs under an atmosphere of sulfur gas,3 or passivating the surface with strongly-coordinating ligands to give the NCs air stability.5 These approaches improve the stability and surface phase purity of pyrite, but may compromise the crystallinity of the NCs,20 or inhibit charge transport.21 A different surface passivation strategy is thus desirable. The use of an inorganic shell layer is a common and effective approach toward eliminating surface trap states in quantum dots.22 However, this approach has not been used previously for pyrite NCs. Iron selenide is a logical choice as a shell material, as it has a close crystal lattice match to pyrite with potential for epitaxial shell growth, as observed for previous systems.23 Bulk iron selenide has a direct band


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 27

gap of 1.23 eV, near that of pyrite (~1.0 eV), and is also environmentally-friendly with low toxicity.24 The iron selenide shell should prevent the formation of surface defects by passivating the surface of pyrite NCs. In addition, iron selenide is known to exhibit phase-dependent magnetic properties,25,26 and could give pyrite NCs additional magnetic functionality. Here we report the synthesis of core/shell NCs consisting of a crystalline pyrite nanocube core with an iron selenide shell. The NCs were characterized via transmission electron microscopy (TEM) and X-ray diffraction (XRD). Optical properties of the pyrite NCs pre- and post-coating were measured via Raman spectroscopy and UV-vis spectroscopy. Transient absorption spectroscopy (TAS) was used to probe carrier behaviour before and after the iron selenide coating reaction. The conductivity of cubic pyrite NCs before and after the iron selenide shell coating reaction was compared on silicon transistor electrodes. The NCs’ magnetic properties were characterized using a vibrating sample magnetometer both before and after field-cooling. It was found that the iron selenide shell significantly improved the NCs’ resistivity, and resulted in less charge recombination at the surface. The iron selenide shell coating gave the nanoparticles ferrimagnetic character at room temperature, further expanding the potential application of pyrite nanomaterials. 2. Experimental 2.1. Materials: Ferrous chloride, anhydrous (FeCl2, Sigma Aldrich, 98%), sulfur powder (sublimed, BDH Chemicals Ltd), hexadecylamine (Sigma Aldrich, 90%), diphenyl ether (Sigma Aldrich, >99%), ferric acetylacetonate (Fe(acac)3, Sigma Aldrich, 97%) selenium powder (Sigma Aldrich, >99.5%), 1-octadecene (Sigma Aldrich, 90%), ethylene diamine (Sigma Aldrich, >99.5%), chloroform (Sigma Aldrich, >99%), toluene (anhydrous, Sigma Aldrich, 99.8%) and


4

Page 5 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ethanol (Fisher Scientific, analytical grade) were all used as-received from the supplier, without further purification. 2.2. Instrumentation: Transmission electron microscope (TEM) images, energy dispersive X-ray spectroscopic (EDS) data and STEM-EDX data were all taken on a JEOL-2100F instrument, at an operating voltage of 200 kV. Powder XRD measurements were performed using a Pan Analytical X’pert Pro MPD X-ray diffraction System using Cu Kα radiation. UV-vis spectroscopic analysis was performed using a Varian Cary 50 Bio UV-vis spectrophotometer. Raman measurements were performed in the spectral range 300-600 cm-1, using the 514.5 nm wavelength of an Ar+ laser on a LabRam HR800 spectrometer. A grating of 1800 cm-1 was used and the laser power was limited to 2.0 mW. Current-voltage characteristics were obtained in the range from -2 V to 2 V using a Keithley SCS 4200. Magnetic measurements were conducted using a Quantum Design Physical Properties Measurement System (PPMS) instrument. 2.3. Synthesis of pyrite nanocubes: FeCl2, anhydrous (128 mg, 1 mmol) and hexadecylamine (HDA, 18.2 g) were combined in a three-necked flask equipped with a thermoprobe. The mixture was heated to 120 °C and degassed under vacuum for 30 min. The mixture was backfilled with nitrogen, and the temperature held at 120 °C. In a separate flask, a solution containing sulfur powder (192 mg, 6 mmol) was dissolved in diphenyl ether (10 mL) was heated to 70 °C and degassed under vacuum for 30 min. After backfilling the flask containing the sulfur solution with nitrogen, the solution was injected into the flask containing the FeCl2/HDA solution at 120 °C. The temperature dropped to 110 °C, was raised to 220 °C and held for 35 min. After this time, the flask was cooled to 50 °C, and 16 mL of raw reaction solution set aside for the iron selenide overgrowth stage.


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 27

2.4. Synthesis of dendritic pyrite nanocrystals: FeCl2, anhydrous (64 mg, 0.5 mmol) and hexadecylamine (18.2 g) were combined in a three-necked flask equipped with a thermoprobe. The mixture was heated to 120 °C and degassed under vacuum for 30 min. The mixture was backfilled with nitrogen, and the temperature held at 120 °C. In a separate flask, a solution containing sulfur powder (144 mg, 4.5 mmol) was dissolved in diphenyl ether (10 mL) was heated to 70 °C and degassed under vacuum for 30 min. The injection, aging and purification steps were carried out in the same manner as for pyrite nanocubes. 2.5. Synthesis of pyrite/iron selenide core/shell nanocrystals: Selenium powder (79.7 mg, 1.0 mmol) was suspended in 1-octadecene (ODE, 21 mL), the mixture heated to 120 °C in a threenecked flask and degassed for 30 min under vacuum. The mixture was then heated to 320 °C to dissolve the selenium, until the solid dissolved and the solution turned yellow. The solution was cooled to 120 °C, at which point 16 mL of the raw reaction solution containing pyrite nanocrystals was injected. The mixture was degassed for a further 30 min under nitrogen at this temperature. Meanwhile, Fe(acac)3 (177 mg, 0.5 mmol) was dissolved in HDA (4.6 g) at 70 °C, and degassed under vacuum for 30 min. The solution was injected into the selenium/pyrite seed solution at 220 °C, and held for 20 min at this temperature. The solution was immersed in a water bath to quench the reaction by quickly returning the reaction solution to room temperature. The reaction product was stored under ambient conditions as a raw reaction solution. 2.6. Transient absorption spectroscopy measurements: Excited state dynamics were studied using ultrafast transient absorption (TA) spectroscopy, in which 400 nm excitation (pump) pulses were generated from the second harmonic of an amplified Ti-sapphire 800 nm laser (SpectraPhysics Spitfire, 100 fs pulsewidth, 3 kHz) and were chopped at half of the amplifier rep-rate. A portion of the 800 nm output was focused in to 3 mm YAG crystal for near-infrared and linearly


6

Page 7 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


translated 3 mm CaF2 crystal window for visible measurements, the generated white light supercontinua were polarized at the magic angle with respect to the pump and used to probe the samples. After transmission through a sample the probe was spatially dispersed using an optical glass prism and read out at 3 kHz using a linear CMOS photodiode array (Imaging Solutions Group) for visible and linear IR photodiode array (Entwicklungsbuero Stresing) for near-infrared measurements. The pump-probe delay was varied using a retroreflector mounted on a computer controlled mechanical delay stage providing a delay range of up to 3 ns. Temporal experimental resolution was limited by ∼200 fs total instrument response function. Approximately 2000 shots were averaged at each time point. Transient absorption (ΔT/T) spectra were obtained according to [T*(λ,t)-To(λ)]/To(λ), where T* and To are transmitted probe intensities measured with the excitation beam unblocked and blocked by the chopper respectively. Sample dispersions for room temperature spectroscopic measurements were contained in a 1 mm path length fused quartz cuvette. 2.7. Electrical measurements: Pre-patterned Au electrodes were prepared on a Si chip (1 cm x 1 cm) with a 300 nm SiO2 thermal oxide. The substrate was cleaned by sonication in acetone and followed by rinsing in isopropanol and subsequently dried with nitrogen. Afterwards, the sample is emerged for 30 min into a 1 % v/v solution of 3-aminopropyltrimethoxysilane in anhydrous toluene in a nitrogen glovebox, in order to improve the adhesion of the photoresist AZ1518 on the silicon oxide surface. Photolithography is performed using a Karl Suss MJB3 UV lamp mask aligner. The 5 nm Cr / 30 nm Au electrodes are deposited using the Angstrom Engineering Nexdep Evaporator and patterned through the application of a lift-off process resulting in 2 mm long parallel Au electrodes (W) with a separation distance (L) of 10 µm.


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 27

2.8. Thin film preparation: To prepare colloidal inks for deposition on substrates, the pyrite and pyrite/selenide nanocrystals were suspended in hot ethanol, and retrieved via centrifugation at 4,000 rpm. The nanoparticles were washed once in toluene to remove excess HDA ligands, centrifuged again, and suspended in chloroform as a concentrated solution. Aggregates were removed via centrifugation at 1,000 rpm. Five drops of solution was cast onto pre-patterned Au electrodes (W = 2000 µm; L = 10 µm), and spin-cast at 1,000 rpm. Ligand exchange to remove the insulating HDA ligand was performed by adding 3-5 drops of ethylene diamine (EDA) onto the substrates and leaving for 30 s, followed by spinning for 10 s at 1,000 rpm, washing the substrate in ethanol and drying under nitrogen flow. The deposition and ligand exchange process was repeated 10 times in total. 2.9. Magnetic measurements: For magnetic measurements, a portion of chloroform solution containing nanocrystals (washed) was dried under vacuum, and the powder (3-5 mg) weighed into a gelatin capsule. The capsule was sealed with non-magnetic tape, and inserted into a Quantum Design Physical Properties Measurement System (PPMS) equipped with a vibrating sample magnetometer (VSM). Results & Discussion Pyrite nanocubes were synthesized via a method adapted from Yoder et al. with significant modifications (see Experimental section in Supporting Information).10 The pyrite seeds had an average size of 82 ± 18 nm as measured from TEM images (Figure 1A). Using the same synthetic method with a lower concentration of iron/sulfur reagents gave similarly-sized (67 ± 16


8

Page 9 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. TEM images are shown of A) as-synthesized pyrite nanocubes and B) pyrite/selenide core/shell nanocubes. C) XRD patterns taken from the pyrite seeds and the core/shell product, with reflections indexed to pyrite (FeS2) and FeSe. D-F) STEM-EDX analysis of FeS2/FeSex nanocubes. G) HRTEM analysis of the core (top) and shell (bottom) of a FeS2/FeSex core/shell NC, with FFT spectra indicated. nm) dendritic pyrite NCs as a reaction product (Figure S1). An iron selenide shell was grown on the surface of pyrite nanocubes via heating the nanocubes in a solution containing Fe(acac)3 and selenium powder (see Experimental section in Supporting Information). Similar reagents were previously used in order to synthesize FeSex NCs by Mao et al.27 A TEM image of the


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 27

FeS2/FeSex core/shell NCs is shown in Figure 1B, and the morphology and size appear to be largely unchanged relative to the pyrite nanocubes. The size distribution histograms (Figure S2) showed only a minor change in NC size, with an average size increase to 87 ± 21 nm. From these measurements, the shell thickness is calculated as 2.5 ± 1.5 nm. XRD analysis of the NCs before and after shell coating (Figure 1C) shows that the pyrite seeds are a perfect match to pyrite, and free of additional peaks corresponding to potential phase impurities of FeSx (Figure S3). The pattern corresponding to the core/shell NCs shows retention of the pyrite reflections, as well as additional reflections at 33.8 and 43.8°. The additional reflections closely match those previously observed for the hexagonal NiAs-type β-FeSe phase of iron selenide.28 The peaks assigned to NiAs-type FeSe are slightly shifted to higher 2° values, relative to the reference peaks. Due to the small calculated size of the FeSex layer, a high level of strain would be expected to be observed, which may explain this shift. Figure 1D-F shows scanning TEM energy-dispersive Xray (STEM-EDX) analysis of the core/shell NCs. A localization of selenium around the cube edges suggests shell localization of the FeSex phase. EDS analysis (Figure S4) showed an average Fe:S:Se atomic ratio of 36:62:2, indicating that the iron selenide shell is very thin compared to the pyrite core. The HRTEM image of the reaction product (Figure 1G, fast Fourier transformed images inset) further confirms the core/shell composition of the material, with crystal planes indexed to pyrite and to iron selenide indicated in the top and bottom FFT spectra, respectively. The assignment of the (102) plane is in good agreement with the XRD data (Figure 1C) showing the FeSe phase of iron selenide. The FeSex phase may grow epitaxially on the pyrite surface due to the close lattice parameter of hexagonal FeSe compared to that of pyrite along the c axis (5.6053 Å and 5.4179 Å, respectively).29


10

Page 11 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The potential for homogeneous nucleation of FeSex nanocrystals in this experiment was considered. In the work by Lee and co-workers that the FeSex shell synthesis was based upon, the reaction temperature was set as 330 °C, and FeSe and FeSe2 nanoparticles were produced.27 In the system using FeS2 NC “seeds” in this report, the reaction temperature was lowered to 220 °C, in order to prevent the homogeneous nucleation of FeSex nanoparticles. It is known that heterogeneous nucleation of shell layers on nanocrystals may take place at lower temperatures than for pure homogeneous growth, due to the energy barrier for nucleation being reduced by the presence of seed structures providing nucleation sites.30,31 Smaller FeSex nanoparticles were not observed in the reaction product after shell growth in this research, from TEM analysis of multiple areas. The NCs appear blue-grey in chloroform solution (Figure 2A). Optical characterization of the nanomaterials was carried out, in order to probe information on the optoelectronic properties. Raman analysis (Figure 2B) shows the signals produced from the S-S vibrations of pyrite nanocubes, at 338 and 372 cm-1, which match well with literature values.32 Analysis of the core/shell NCs showed that the characteristic pyrite signals disappeared, while no bands for tetragonal or hexagonal FeSe were observed.33 The disappearance of pyrite vibrations may be due to diffusion of selenium atoms into the pyrite lattice during the coating reaction, which would significantly reduce the energy of the S-S vibrations near the surface. Alternatively, this effect may be due to the disappearance of surface optical phonons, which can occur as the shell layer of core/shell NCs increases in relative composition to the core.34 The pattern produced from pyrite NCs heated to 220 °C for 20 min is shown for comparison, to confirm that the disappearance of the Raman signals is not simply an effect of aging the pyrite at elevated temperatures, and also that the phase purity of iron pyrite remains after the aging process.


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 27

The steady-state absorption spectra of the FeS2 and FeS2/FeSex NCs are shown in Figure S5, showing strong absorption features in the visible region. An absorption tail extending into the near-infrared (NIR) is also observed for each NC type. The broadness of these absorption tails is likely due to scattering of light, taking into account the large size of these nanocrystals, consistent with the observation made previously by Alivisatos and co-workers.35 The scattering features make these steady-state spectra unsuitable for band gap estimation of the NCs. In order to accurately determine band gap via Tauc plot analysis, an integrating sphere was used to isolate absorption from the extinction spectra (the apparatus is described in further detail elsewhere).36 The absorption spectra are illustrated in Figure 2C. The Tauc plot (inset) shows that the direct band gap of the pyrite is effectively unchanged after the FeSex shell coating reaction. The known band gap sizes for pyrite and iron selenide are very close (1.0 and 1.2 eV, respectively).3,27 Due to quantum confinement effects, the very small (80 nm pyrite core is outside of the quantum confinement size regime, and the probability of internal defects is large, leading to primarily non-radiative decay.37


12

Page 13 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. A) Colour photographs of dilute chloroform suspensions of FeS2 and FeS2/FeSex core/shell NCs. B) Raman spectra for FeS2 NCs before and after shell coating. The spectrum for pyrite NCs heated to 200 C is included for comparison. C) Absorption measurements taken via an integrating sphere setup, with the scattering of light eliminated from the spectrum. D) TA spectra of FeS2 and FeS2/FeSex core/shell NCs, as well as dendritic FeS2 NCs, probed at t = 12.5 ps. The data presented are smoothed using LOESS regression (un-smoothed data shown in Figure S7). TAS was used to examine photogenerated carrier behaviour of pyrite NCs before and after the coating reaction (see Supporting Information for detailed description of the setup used). TAS measurements carried out on pyrite nanocubes showed a short lived +ΔT/T photobleaching feature peaking at 720 nm (see Figure 2D, Figure S6), which indicates ground state transition bleaching of pyrite.3 Rather than the band gap transition, this likely represent the 3d t2g à eg* electronic transition, in accordance with previous observations which predict this transition occurring at 1.7 eV.38,39 The lower-energy end of the TAS spectrum is dominated by a long lived


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 27

broadband photoinduced absorption (PIA) signal (-ΔT/T) peaking at 890 nm (Figure 2D, “FeS2 nanocubes”). The low energy feature arises from photoexcited carrier absorption in pyrite trap states, and is therefore expected to heavily depend on the defects and p-type vacancies, largely localized at the surface of pyrite NCs.3,13 As the number of S vacancy defects should be proportionate to pyrite NC surface area, the dendritic pyrite NCs were expected to have a stronger defect influence. A similar profile broadband PIA signature was also observed in the dendritic NC TAS spectra, however it was blue-shifted relative to the pyrite nanocubes with smaller surface area (Figure 2D, “FeS2 dendritic”). This observation suggests dominant defect state contribution to the PIA feature, where more significant defect state population results in spectral shift to higher energies. In the FeS2/FeSex NC TAS spectrum (Figure 2D, “FeS2/FeSex”), the ground state bleaching feature remains in the same position as in uncoated pyrite nanocubes indicating the same band edge carrier behaviour. In contrast, broadband PIA signature shows a pronounced red-shift to lower energies peaking at around 1030 nm. The red-shifted position of the PIA indicates a reduction of the pyrite defect site influence on the photogenerated carriers in core/shell NCs. The observation further confirms that the NCs’ pyrite-based surface defects have been significantly reduced by the iron selenide coating reaction, resulting in a reduction of active trap states. The effect of shell coating on the electrical properties of the NCs was examined. Figure 3 shows the I-V curves of dark current produced from FeS2 and FeS2/FeSex core/shell NCs spin-cast on pre-patterned gold electrodes after ligand exchange with ethylene diamine (EDA). Based on the linear fit of the I-V characteristic, the electrode geometry (scheme in Figure S8) and a film thickness of 150 nm, a resistivity for the FeS2 NC film of 3.5 x 103 Ω cm was determined. By contrast, the FeS2/FeSex NC film showed a resistivity of 2.0 x 104 Ω cm, as well as a weak


14

Page 15 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


rectifying behaviour. The differential conductance dependence on the applied voltage (Figure 3, inset) also indicates a significant change of the electronic properties due to the shell coating. Previously, the defect-

Figure 3. I-V measurements of spin-cast thin films from FeS2 and FeS2/FeSex NCs, after ligand exchange with EDA. The inset shows differential conductance with applied voltage. induced high surface conductivity of pyrite NCs has been ascribed as a main factor impeding their performance in PV devices.18 Due to the device structure used in this research, where the electrical contacts are 10 μm apart, significant photoactive charge harvesting is not plausible, and ON/OFF behaviour such as that in photovoltaic cells would not be expected to be observed. However, the increased resistivity contributed by the iron selenide shell coating could significantly decrease leakage current associated and increase efficiency of pyrite-based PV devices such as Schottky or p-n heterojunction devices.6,18 As iron selenide is known to be ferrimagnetic, depending on phase and stoichiometry,25 the change in magnetic behaviour of the core/shell NCs was examined. Figure 4A shows the


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 27

magnetization (M) versus applied field (H) for FeS2 and FeS2/FeSex NCs at 300 K. The FeS2 NCs show a diamagnetic response, with a negative magnetic susceptibility (χ). By contrast, the FeS2/FeSex core/shell NCs show a positive χ and ferrimagnetic response, with M increasing up to 0.34 emu g-1 at 60 kOe, and a coercive field (Hc) of 1037 Oe. Both the Ms and Hc values are smaller than those observed previously for hexagonal-phase nanocrystalline iron selenide40 or bulk FeSex,41 which may be due to the very small volume fraction of the FeSex shell layer on the pyrite NCs. A comparison of the magnetic properties of the FeS2/FeSex NCs at 300 K and

Figure 4. M/H measurements at 300 K for FeS2 and FeS2/FeSex core/shell NCs, showing diamagnetic and ferrimagnetic response respectively at high fields. B) Temperature-dependent M/H measurements for FeS2/FeSex NCs (low-field measurements inset). after field-cooling at 10 K is shown in Figure 4B. At 10 K, the M/H curve does not flatten even at high fields, showing paramagnetism at this temperature. Given the large surface-to-volume ratio of the FeSex phase, this paramagnetic behaviour can be explained by canting of magnetic spins near the FeSex-air interface.42 In addition, analysis of the M/H curve at low fields (Figure 4B, inset) shows double-loop behaviour at 10 K, where the magnetization increases sharply up to ~200 Oe then at a lower rate up to 40 kOe. This magnetic behaviour is characteristic of ferrimagnetic NCs with a spin-glass surface phase.43 It is notable that this double-loop behaviour


16

Page 17 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


was not previously observed for FeSex nanomaterials. At 10 K, the Hc value is measured as 1084 Oe, which is not significantly different from that measured at 300 K. As Hc appears to be independent of temperature, the main factor contributing to hysteresis is assigned to be the pinning of magnetic spins near the FeS2-FeSex interface, in accordance with the effect previously observed for core/shell nanoparticles.44 In this case, the energy required to overcome this pinning effect is larger than the thermal energy available at room temperature, hence the very similar Hc at 10 K and 300 K. The core/shell NCs are able to be separated from solution via aggregation near an external magnet (Figure S9). FeS2 NCs suspended in chloroform showed no aggregation near an external magnetic after direct exposure for 1 hr in the same manner as for core/shell nanocrystals. This is to be expected for diamagnetic nanocrystals with a negative χ value. The magnetic attraction of the core/shell NCs to an external magnet could serve to simplify purification procedures. In addition, recent research has shown that a magnetic field can be used to manipulate photocurrent generation in devices containing a magnetic component, through enhanced rectification of photocurrent in p-n heterojunctions.16,45 Bi-functional magneto-optoelectronic NCs are a growing area of interest, with systems including CdFeS2 and CuInS2 having been explored in recent reports.16,46 The magnetic properties of these core/shell NCs also provides opportunity in applications such as biomedicine, where iron selenide nanocrystals have recently shown utility.24,47 The NCs’ combination of ferrimagnetic response and high optical absorption in the near-infrared makes them ideal candidates for dual-functional magnetic resonance imaging and photothermal cancer therapy mediators.48,49


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 27

Conclusions In conclusion, a method to synthesize FeS2/FeSex core/shell NCs was developed. The reaction formed a 2.5 nm FeSex shell localized at the edges and faces of pyrite nanocubes. Spectroscopic measurements showed that the direct band gap of pyrite was retained for the core/shell NCs. The iron selenide coating procedure significantly reduced carrier trapping on the NCs’ surface, evidenced by TAS measurements showing a pronounced red-shift of defect-induced photoexcited carrier absorption. Electrical measurements showed that the surface resistivity of the NCs improved by a factor of 5.8 after shell coating, thereby reducing the semi-metallic conductivity of pyrite nanocubes, which should serve to prevent leakage current in pyrite-based PV devices. Magnetic measurements revealed a transition of the magnetic behaviour from diamagnetic to ferrimagnetic, with magnetic moment measured at 0.34 emu g-1 at H = 60 kOe for the core/shell NCs measured at 300 K. The surface-passivation method employed here gives additional functionality to pyrite NCs, and is a step toward realizing the potential of this low-cost material for energy production. The combination of magnetic and optoelectronic properties for the core/shell NCs may also open new avenues for pyrite NCs in magneto-optoelectronics and biomedicine.


18

Page 19 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ASSOCIATED CONTENT Supporting Information. TEM and XRD characterization of branched pyrite NCs, size distribution histograms, XRD data, EDS data, steady-state visible-NIR spectra, TAS surface plots, raw TAS data, scheme of the Au electrode device and photographs of magnetic separation of core/shell NCs are all available as Supporting Information.

AUTHOR INFORMATION Present address ‡

School of Chemistry, University of New South Wales, NSW 2052, Australia

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT J. E. H. acknowledges the Royal Society of New Zealand for funding under the Marsden Fund and the Rutherford Discovery Fellowship fund. J. M. H. acknowledges the support of a Rutherford Discovery Fellowship.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

REFERENCES (1)

Wadia, C.; Alivisatos, A. P.; Kammen, D. M. Materials Availability Expands the Opportunity for Large-Scale Photovoltaics Deployment. Environ. Sci. Technol. 2009, 43, 2072-2077.

(2)

Lee, N.; Yoo, D.; Ling, D.; Cho, M. H.; Hyeon, T.; Cheon, J. Iron Oxide Based Nanoparticles for Multimodal Imaging and Magnetoresponsive Therapy. Chem. Rev. 2015, 115, 10637-10689.

(3)

Shukla, S.; Xing, G.; Ge, H.; Prabhakar, R. R.; Mathew, S.; Su, Z.; Nalla, V.; Venkatesan, T.; Mathews, N.; Sritharan, T. et al. Origin of Photocarrier Losses in Iron Pyrite (FeS2) Nanocubes. ACS Nano 2016, 10, 4431-4440.

(4)

Ennaoui, A.; Fiechter, S.; Goslowsky, H.; Tributsch, H. Photoactive Synthetic Polycrystalline Pyrite (FeS2). J. Electrochem. Soc. 1985, 132, 1579-1582.

(5)

Bi, Y.; Yuan, Y.; Exstrom, C. L.; Darveau, S. A.; Huang, J. Air Stable, Photosensitive, Phase Pure Iron Pyrite Nanocrystal Thin Films for Photovoltaic Application. Nano Lett. 2011, 11, 4953-4957.

(6)

Wang, D.-Y.; Jiang, Y.-T.; Lin, C.-C.; Li, S.-S.; Wang, Y.-T.; Chen, C.-C.; Chen, C.-W. Solution-Processable Pyrite FeS2 Nanocrystals for the Fabrication of Heterojunction Photodiodes with Visible to NIR Photodetection. Adv. Mater. 2012, 24, 3415-3420.

(7)

Macpherson, H. A.; Stoldt, C. R. Iron Pyrite Nanocubes: Size and Shape Considerations for Photovoltaic Application. ACS Nano 2012, 6, 8940-8949.

(8)

Zhu, L.; Richardson, B. J.; Yu, Q. Anisotropic Growth of Iron Pyrite FeS2 Nanocrystals via Oriented Attachment. Chem. Mater. 2015, 27, 3516-3525.

(9)

Wadia, C.; Wu, Y.; Gul, S.; Volkman, S. K.; Guo, J.; Alivisatos, A. P. Surfactant-Assisted Hydrothermal Synthesis of Single Phase Pyrite FeS2 Nanocrystals. Chem. Mater. 2009,


20

Page 21 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


21, 2568-2570. (10)

Yoder, T. S.; Cloud, J. E.; Leong, G. J.; Molk, D. F.; Tussing, M.; Miorelli, J.; Ngo, C.; Kodambaka, S.; Eberhart, M. E.; Richards, R. M. et al. Iron Pyrite Nanocrystal Inks: Solvothermal Synthesis, Digestive Ripening, and Reaction Mechanism. Chem. Mater. 2014, 26, 6743-6751.

(11)

Bai, Y.; Yeom, J.; Yang, M.; Cha, S.-H.; Sun, K.; Kotov, N. A. Universal Synthesis of Single-Phase Pyrite FeS2 Nanoparticles, Nanowires, and Nanosheets. J. Phys. Chem. C 2013, 117, 2567-2573.

(12)

Puthussery, J.; Seefeld, S.; Berry, N.; Gibbs, M.; Law, M. Colloidal Iron Pyrite (FeS2) Nanocrystal Inks for Thin-Film Photovoltaics. J. Am. Chem. Soc. 2011, 133, 716-719.

(13)

Seefeld, S.; Limpinsel, M.; Liu, Y.; Farhi, N.; Weber, A.; Zhang, Y.; Berry, N.; Kwon, Y. J.; Perkins, C. L.; Hemminger, J. C. et al. Iron Pyrite Thin Films Synthesized from an Fe(acac)3 Ink. J. Am. Chem. Soc. 2013, 135, 4412-4424.

(14)

Wang, Y.-C.; Wang, D.-Y.; Jiang, Y.-T.; Chen, H.-A.; Chen, C.-C.; Ho, K.-C.; Chou, H.L.; Chen, C.-W. FeS2 Nanocrystal Ink as a Catalytic Electrode for Dye-Sensitized Solar Cells. Angew. Chem. Int. Ed. 2013, 52, 6694-6698.

(15)

Yu, B.-B.; Zhang, X.; Jiang, Y.; Liu, J.; Gu, L.; Hu, J.-S.; Wan, L.-J. Solvent-Induced Oriented Attachment Growth of Air-Stable Phase-Pure Pyrite FeS2 Nanocrystals. J. Am. Chem. Soc. 2015, 137, 2211-2214.

(16)

Gong, M.; Kirkeminde, A.; Xie, Y.; Lu, R.; Liu, J.; Wu, J. Z.; Ren, S. Iron Pyrite (FeS2) Broad Spectral and Magnetically Responsive Photodetectors. Adv. Opt. Mater. 2013, 1, 78-83.

(17)

Cabán-Acevedo, M.; Faber, M. S.; Tan, Y.; Hamers, R. J.; Jin, S. Synthesis and Properties


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

of Semiconducting Iron Pyrite (FeS2) Nanowires. Nano Lett. 2012, 12, 1977-1982. (18)

Steinhagen, C.; Harvey, T. B.; Stolle, C. J.; Harris, J.; Korgel, B. A. Pyrite Nanocrystal Solar Cells: Promising, or Fool’s Gold? J. Phys. Chem. Lett. 2012, 3, 2352-2356.

(19)

Büker, K.; Alonso-Vante, N.; Tributsch, H. Photovoltaic Output Limitation of n-FeS2 (Pyrite) Schottky Barriers: A Temperature-Dependent Characterization. J. Appl. Phys. 1992, 72, 5721-5728.

(20)

Treml, B. E.; Robbins, A. B.; Whitham, K.; Smilgies, D.-M.; Thompson, M. O.; Hanrath, T. Processing–Structure–Property Relationships in Laser-Annealed PbSe Nanocrystal Thin Films. ACS Nano 2015, 9, 4096-4102.

(21)

Stolle, C. J.; Harvey, T. B.; Korgel, B. A. Nanocrystal Photovoltaics: a Review of Recent Progress. Curr. Opin. Chem. Eng. 2013, 2, 160-167.

(22)

Talapin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. Highly Luminescent Monodisperse

CdSe

and

CdSe/ZnS

Nanocrystals

Synthesized

in

a

Hexadecylamine−Trioctylphosphine Oxide−Trioctylphospine Mixture. Nano Lett. 2001, 1, 207-211. (23)

Peng, X.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. Epitaxial Growth of Highly Luminescent CdSe/CdS Core/Shell Nanocrystals with Photostability and Electronic Accessibility. J. Am. Chem. Soc. 1997, 119, 7019-7029.

(24)

Cheng, L.; Shen, S.; Shi, S.; Yi, Y.; Wang, X.; Song, G.; Yang, K.; Liu, G.; Barnhart, T. E.; Cai, W. et al. FeSe2-Decorated Bi2Se3 Nanosheets Fabricated via Cation Exchange for Chelator-Free

64

Cu-Labeling and Multimodal Image-Guided Photothermal-Radiation

Therapy. Adv. Funct. Mater. 2016, 26, 2185-2197. (25)

Hirone, T.; Maeda, S.; Tsuya, N. On the Ferrimagnetism of Iron Selenides. J. Phys. Soc.


22

Page 23 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Japan 1954, 9, 496-499. (26)

Terzieff, P.; Komarek, K. L. The Antiferromagnetic and Ferrimagnetic Properties of Iron Selenides with NiAs-type Structure. Monatsch. Chem. 1978, 109, 1037-1047.

(27)

Mao, X.; Kim, J.-G.; Han, J.; Jung, H. S.; Lee, S. G.; Kotov, N. A.; Lee, J. Phase-Pure FeSex (x = 1, 2) Nanoparticles with One- and Two-Photon Luminescence. J. Am. Chem. Soc. 2014, 136, 7189-7192.

(28)

Sines, I. T.; Schaak, R. E. Phase-Selective Chemical Extraction of Selenium and Sulfur from Nanoscale Metal Chalcogenides: A General Strategy for Synthesis, Purification, and Phase Targeting. J. Am. Chem. Soc. 2011, 133, 1294-1297.

(29)

Kumar, R. S.; Zhang, Y.; Sinogeikin, S.; Xiao, Y.; Kumar, S.; Chow, P.; Cornelius, A. L.; Chen, C. Crystal and Electronic Structure of FeSe at High Pressure and Low Temperature. J. Phys. Chem. B 2010, 114, 12597-12606.

(30)

Park, J.; Lee, E.; Hwang, N.-M.; Kang, M.; Kim, S. C.; Hwang, Y.; Park, J.-G.; Noh, H.J.; Kim, J.-Y.; Park, J.-H. et al. One-Nanometer-Scale Size-Controlled Synthesis of Monodisperse Magnetic Iron Oxide Nanoparticles. Angew. Chem. Int. Ed. 2005, 44 (19), 2873-2877.

(31)

Habas, S. E.; Lee, H.; Radmilovic, V.; Somorjai, G. A.; Yang, P. Shaping Binary Metal Nanocrystals Through Epitaxial Seeded Growth. Nat. Mater. 2007, 6, 692-697.

(32)

Ushioda, S. Raman Scattering from Phonons in Iron Pyrite (FeS2). Solid State Commun. 1972, 10, 307-310.

(33)

Gnezdilov, V.; Pashkevich, Y. G.; Lemmens, P.; Wulferding, D.; Shevtsova, T.; Gusev, A.; Chareev, D.; Vasiliev, A. Interplay Between Lattice and Spin States Degree of Freedom in the FeSe Superconductor: Dynamic Spin State Instabilities. Phys. Rev. B


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

2013, 87, 144508. (34)

Mukherjee, P.; Lim, S. J.; Wrobel, T. P.; Bhargava, R.; Smith, A. M. Measuring and Predicting the Internal Structure of Semiconductor Nanocrystals through Raman Spectroscopy. J. Am. Chem. Soc. 2016, 138, 10887-10896.

(35)

Lucas, J. M.; Tuan, C.-C.; Lounis, S. D.; Britt, D. K.; Qiao, R.; Yang, W.; Lanzara, A.; Alivisatos, A. P. Ligand-Controlled Colloidal Synthesis and Electronic Structure Characterization of Cubic Iron Pyrite (FeS2) Nanocrystals. Chem. Mater. 2013, 25, 16151620.

(36)

Darby, B. L.; Auguié, B.; Meyer, M.; Pantoja, A. E.; Le Ru, E. C. Modified Optical Absorption of Molecules on Metallic Nanoparticles at Sub-Monolayer Coverage. Nat. Photonics 2015, 10, 40-45.

(37)

Gong, M.; Kirkeminde, A.; Ren, S. Symmetry-Defying Iron Pyrite (FeS2) Nanocrystals through Oriented Attachment. Sci. Rep. 2013, 3, 1174.

(38)

Khan, M. A. Energy Bands in Pyrite-Type Crystals. J. Phys. C Solid State Phys. 1976, 9, 81-94.

(39)

Schlegel, A.; Wachter, P. Optical Properties, Phonons and Electronic Structure of Iron Pyrite (FeS2). J. Phys. C Solid State Phys. 1976, 9, 3363-3369.

(40)

Zhang, H.; Long, G.; Li, D.; Sabirianov, R.; Zeng, H. Fe3Se4 Nanostructures with Giant Coercivity Synthesized by Solution Chemistry. Chem. Mater. 2011, 23, 3769-3774.

(41)

Takahashi, T.; Kuno, S.; Honda, N.; Takemura, Y.; Kakuno, K.; Saito, K. Structural and Magnetic Properties of FexSey Thin Films During Their Selenization Process. J. Appl. Phys. 1998, 83, 6533-6535.

(42)

Martínez-Boubeta, C.; Simeonidis, K.; Angelakeris, M.; Pazos-Pérez, N.; Giersig, M.;


24

Page 25 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Delimitis, A.; Nalbandian, L.; Alexandrakis, V.; Niarchos, D. Critical Radius for Exchange Bias in Naturally Oxidized Fe Nanoparticles. Phys. Rev. B 2006, 74, 054430. (43)

Cozzoli, P. D.; Snoeck, E.; Garcia, M. A.; Giannini, C.; Guagliardi, A.; Cervellino, A.; Gozzo, F.; Hernando, A.; Achterhold, K.; Ciobanu, N. et al. Colloidal Synthesis and Characterization of Tetrapod-Shaped Magnetic Nanocrystals. Nano Lett. 2006, 6, 19661972.

(44)

Wen, T.; Krishnan, K. M. Magnetic Properties of Aucore-Coshell Nanoparticles. J. Appl. Phys. 2011, 109, 07B515.

(45)

Bera, A.; Pal, A. J. Aligned Magnetic Domains in p- and n-Type Ferromagnetic Nanocrystals and in pn-Junction Nanodiodes. ACS Appl. Mater. Interfaces 2013, 5, 12083-12088.

(46)

Rice, W. D.; McDaniel, H.; Klimov, V. I.; Crooker, S. A. Magneto-Optical Properties of CuInS2 Nanocrystals. J. Phys. Chem. Lett. 2014, 5, 4105-4109.

(47)

Fu, T.; Chen, Y.; Hao, J.; Wang, X.; Liu, G.; Li, Y.; Liu, Z.; Cheng, L. Facile Preparation of Uniform FeSe2 Nanoparticles for PA/MR Dual-Modal Imaging and Photothermal Cancer Therapy. Nanoscale 2015, 7, 20757-20768.

(48)

Kim, J.; Park, S.; Lee, J. E.; Jin, S. M.; Lee, J. H.; Lee, I. S.; Yang, I.; Kim, J.-S.; Kim, S. K.; Cho, M.-H. et al. Designed Fabrication of Multifunctional Magnetic Gold Nanoshells and Their Application to Magnetic Resonance Imaging and Photothermal Therapy. Angew. Chem. Int. Ed. 2006, 45, 7754-7758.

(49)

Bardhan, R.; Chen, W.; Perez-Torres, C.; Bartels, M.; Huschka, R. M.; Zhao, L. L.; Morosan, E.; Pautler, R. G.; Joshi, A.; Halas, N. J. Nanoshells with Targeted Simultaneous Enhancement of Magnetic and Optical Imaging and Photothermal


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

Therapeutic Response. Adv. Funct. Mater. 2009, 19, 3901-3909.


26

Page 27 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC figure


27

Iron Selenide

Recommend Documents