Magnet-in-the-Semiconductor Nanomaterials: High Electron Mobility

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Letter

Magnet-in-the-Semiconductor Nanomaterials: High Electron Mobility in AllInorganic Arrays of FePt/CdSe and FePt/CdS Core-Shell Heterostructures Jae Sung Son, Jong-Soo Lee, Elena V. Shevchenko, and Dmitri V. Talapin J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/jz400612d • Publication Date (Web): 21 May 2013 Downloaded from http://pubs.acs.org on May 22, 2013

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The Journal of Physical Chemistry Letters

Magnet-in-the-Semiconductor Nanomaterials: High Electron Mobility in All-Inorganic Arrays of FePt/CdSe and FePt/CdS Core-Shell Heterostructures Jae Sung Son, †,⊥ Jong-Soo Lee,†,‡,⊥ Elena V. Shevchenko,§ and Dmitri V. Talapin*,†,§

† ‡

Department of Chemistry, University of Chicago, Chicago, IL 60637, United States

Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu, 711-873, South Korea

§Center

for Nanoscale Materials, Argonne National Lab, Argonne, IL 60439, United States

ACS Paragon Plus Environment

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ABSTRACT We report a colloidal synthesis, electrical and magnetotransport properties of multifunctional “magnet-in-the-semiconductor” nanostructures composed of FePt core and CdSe or CdS shell. Thin films of all-inorganic FePt/CdSe and FePt/CdS core-shell nanostructures capped with In2Se42- molecular chalcogenide (MCC) ligands exhibited n-type charge transport with high field-effect electron mobility of 3.4 cm2/V·s and 0.02 cm2/V·s, respectively. These nanostructures also showed a negative magnetoresistance characteristic for spin-dependent tunneling. We discuss the mechanism of charge transport and gating in the arrays of metal/semiconductor core-shell nanostructures.

TOC

KEYWORDS. FePt, CdSe, core-shell nanostructures, field-effect transistor, magnetoresistance.


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Colloidal semiconductor nanocrystals (NCs) have been extensively utilized in various technological areas1 due to their controllable optical and electronic properties.2 Recently, arrays of semiconductor NCs have shown great potential for light-emitting diodes,3 photovoltaic devices,4 field-effect transistors (FETs),5 and thermoelectric devices.6 In such system, the strong electronic coupling between individual NCs, which mainly depended on the characteristics of the inter-particle medium, was essential to guarantee the high electrical conduction in NC devices. The recent successful developments of various ligands such as inorganic molecular metal chalcogenide complexes (MCCs),7 hydrazine,5 and ammonium thiocyanate8 for decreasing the inter-particle distance in NC solids improved the charge transport via strong electronic coupling, eventually enhancing device performance. For example, In2Se42- MCC-capped CdSe NC thin films exhibited the field-effect electron mobility of ~16 cm2 V-1s-1 at room temperature.9 Hybrid nanostructures containing two or more disparate components have been considered as a platform to design multifunctional properties coming from each constituent,10 and create new synergistic properties. Recently, our group reported first “magnet-in-the-semiconductor” FePtPbX (X=S and Se) hetero-nanostructures, showing interesting charge transport and magnetic properties.11 Furthermore, the control of the spin in these materials was demonstrated by large magnetoresistance, which can find important technological applications in magnetic storage12 and spintronics.13 Cadmium chalcogenides represent the most studied family of nanostructured materials and their surface chemistry, electronic and optical properties are well understood.2 In this work, we synthesized cadmium chalcogenide-based “magnet-in-the-semiconductor” nanostructures, combined them with MCC ligands and studied electrical and magnetotransport properties of obtained multifunctional nanostructure arrays.


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The FePt/CdSe core-shell nanostructures were synthesized by the injection of selenium suspension in 1-octadecene into a reaction mixture containing monodisperse FePt NCs, 1,2tetradecanediol, and Cd(acac)2 dissolved in 1-octadecene. FePt NCs with the size of 3.0 nm were synthesized according the procedure described in Ref.14 and used as the seed NCs for heterogeneous nucleation of CdSe phase. Obtained FePt/CdSe nanostructures had nearly spherical shape with the size of ~ 11 nm and exhibited relatively narrow size distribution with standard deviation of less than 15% (Figure 1a). FePt/CdS core-shell nanostructures were produced by similar synthetic route by using sulfur suspension in 1-octadecene as the sulfur precursor. Obtained nanostructures also had a spherical shape with ~10 nm size, as shown in Figure 1b. The high-resolution transmission electron microscopy (HRTEM) images of FePt/CdSe and FePt/CdS nanostructures (Insets of Figure 1a and b) revealed crystalline nature of these nanostructures. The XRD patterns of FePt/CdSe and FePt/CdS nanostructures (Figure 2a) showed the patterns of both FePt core phase, and CdSe and CdS shell phases, which corresponded to the fcc FePt structure, and hexagonal wurtzite CdSe and CdS structures, respectively. The surface of as-synthesized FePt/CdX (X=S and Se) nanostructures was coated by the organic ligands. To prepare all-inorganic nanostructures, organic ligands coating on the surfaces of nanostructures were exchanged with inorganic In2Se42- MCC ligand prepared by roomtemperature dissolution of bulk In2Se3 in hydrazine in the presence of additional elemental Se, which adopted In2Se42- structure according to Mitzi et al.15 In 2011 our group reported that In2Se42- MCC ligands enabled high electron mobility in CdSe NC arrays.9 TEM and XRD analysis confirmed that the ligand exchange procedure did not cause any changes in the size and shape of nanostructures (Figures 1c and d, and 2a). The Fourier transform infrared (FT-IR)


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absorption spectroscopy revealed the disappearance of the C-H stretching modes (2,700-3,000 cm-1) in the IR in modified nanostructures (Figure S1) that was indicative of complete replacement of the organic ligands by inorganic MCCs. The absorption spectrum of organicscapped FePt/CdSe and FePt/CdS nanostructures showed broad excitonic transitions near ~650 nm and ~500 nm, respectively (Figure 2b). The In2Se42- MCC-capped nanostructures also exhibited similar absorption spectra, which further confirmed no change of the size of nanostructures after the ligand exchange (Figure 2b). The broader features in absorption spectra of FePt/CdX (X=S, Se) as compared to the adsorption spectra of monodisperse CdSe and CdS quantum dots can be attributed to broader size distribution of our multicomponent nanostructures. In addition, the presence of metal core can homogeneously broaden the excitonic transition due to the coupling to the continuum of energy states in metal core nanoparticles.11, 16 The magnetic properties of FePt, FePt/CdSe, and FePt/CdS nanostructures were characterized using a superconducting quantum interference device magnetometer (SQUID). The hysteresis loop of as-synthesized FePt NCs (Figure S2a) showed superparamagnetic response at 300 K and ferromagnetic properties with the coercive field (Hc) of ~1.0 kOe at 5 K, which is the typical behavior of fcc-phase FePt NCs.17 The saturation magnetization of FePt NCs was 0.8 emu/g at 300 K, which was much lower than the value of 6.8 emu/g at 5 K. The blocking temperature (Tb) of FePt NCs estimated by the peak in the zero-filed-cooled (ZFC) curve (Figure S2d) was 15 K, which was identical to that of the reported fcc-structured FePt NCs with the similar size.17 The Tb of both FePt/CdSe and FePt/CdS nanostructures were 15 K, same as the value of FePt NCs, which reveals that the magnetic properties of core-shell nanostructures were dominated by the FePt core (Figure S2d). The saturation magnetizations of FePt/CdSe and FePt/CdS nanostructures at 5 K (Figures S2b and c) decreased to 2.6 emu/g and 2.5 emu/g, which could be


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understood considering the presence of nonmagnetic CdSe and CdS phase in the core-shell nanostructures. However, Hc of FePt/CdSe and FePt/CdS nanostructures also significantly decreased to ~10 Oe at 5 K, even below Tb (Figures S2b and c), which is somewhat counterintuitive because the magnetic properties of current core-shell nanostructures were dominated by FePt core NCs. Previous research reveals that the magnetic properties of FePt NCs strongly depend on the surface rather than interior of the particles.18, 19 The numerical calculation showed that a very thin layer (~0.5 nm) of magnetically soft shell could significantly reduce the coercivity of FePt NCs and this effect was intensified with the decrease of FePt NC size.19 In our core-shell nanostructures, the size of the magnetic FePt seed was 3.0 nm and we assumed that the formation of a very thin soft phase of iron chalcogenide at interface of magnetic FePt core and semiconductor CdSe or CdS shell significantly reduced the coercivity of the core-shell nanostructures. Similar behavior was reported in FePt/PbS and FePt/PbSe core-shell nanostructures due to the formation of soft phase of Fe-S and Fe-Se layers at interfaces.11 To study charge transport, the field effect transistor (FET) properties in nanostructure arrays were measured in the back-gate geometry (Figure 3a). The thin films of all-inorganic In2Se42MCC-capped FePt/CdSe and FePt/CdS nanostructures were prepared by spin-coating of nanostructures onto doped silicon wafers with a 100 nm oxide layer. The films were further annealed at 200 oC for 1 h. After heat treatment, no change in volume and thickness were detected. The FT-IR spectrum of the annealed film (Figure S1) showed no observable peaks, which indicate that N2H4 and N2H5+ species were evaporated during the heat treatment. The XRD patterns of In2Se42- MCC-capped FePt/CdSe and FePt/CdS nanostructures annealed at 200 o

C for 1 h (Figure 2a) were almost identical to those of as-synthesized nanostructures capped by

the organic ligands. The absorption spectra of In2Se42- MCC-capped FePt/CdSe and FePt/CdS


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nanostructure thin films annealed at 200 oC (Figure S3) showed small broadening and red shift of the first excitonic peaks, which could resulted from the stronger electronic coupling between nanostructures during annealing. The aluminum source (S) and drain (D) electrodes were deposited onto the annealed films of In2Se42- MCC-capped FePt/CdSe and FePt/CdS nanostructure by thermal evaporation. Low work function aluminum electrodes (φAl ~4.08 eV) are known to efficiently inject charge carriers into conduction states of CdSe and CdS nanoparticles. In both FePt/CdSe and FePt/CdS nanostructures arrays, the drain current (ID) was modulated by applying the gate voltage (VG), as shown in Figures 3b and 3d. In FePt/CdSe nanostructures, ID increased with the increase of VG at constant VDS of 2 V, which is the typical characteristic of the n-type transport (Figure 3c). The Ion/Ioff ratio was ~1×102. The field effect mobility of this film in the linear regime was evaluated by the slope of the ID vs VG plot. The estimated field-effect mobility (µe) reached ~3.4 cm2/V·s, which is ~40 times higher than that of previous “magnet-inthe-semiconductor” system of FePt-PbSe hybrid nanostructures bridged by hydrazine molecules.11 This large increase of the field effect mobility demonstrates the strong exchange coupling between individual MCC-capped FePt/CdSe nanostructures. The FET devices of FePt/CdS nanostructures also showed n-type transport (Figures 3d and e). Their field-effect mobility in the saturation regime was 0.02 cm2/V·s as extracted from the slope of ID1/2 vs VG and the Ion/Ioff ratio was ~1×103. To understand the mechanism of charge transport in the arrays of FePt/CdSe core-shell nanostructures, one has to know alignment of the energy levels in the core and the shell. Reported work function of bulk FePt alloy (φFePt) is about 5.0 eV.20 This value can vary


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depending on the alloy composition. The NC surface can also have a strong effect on the absolute positions of the energy levels. For example, electron donating surface ligands or ions can shift the energy levels of CdSe NCs in the cathodic direction.21 There is a large spread in the reported values for the position of 1Se state of CdSe NCs. The values reported by different groups for ~5 nm diameter CdSe NCs vary between -4.7 eV22 and -3.3 eV21 with respect to the vacuum level. This precludes us from making any quantitative predictions for exact positions of the energy levels in the shell and the core of FePt/CdSe nanostructures. The core-shell morphology also possesses a challenge for direct experimental measurement of the energy level alignment between FePt core and CdSe shell. At the same time, all reported data suggest the qualitative picture where the 1Se state of CdSe shell lies above the highest occupied state in the FePt core as shown in Figure 3f. We apparently do not know the energy difference (∆E) between the highest occupied state in FePt core and the 1Se state of CdSe shell but based on published data, it should be some hundreds meV. The observation of gated transport with high electron mobility in FePt/CdSe nanostructures appears counterintuitive because metal cores should efficiently trap electrons from semiconducting shell and pin the Fermi level. We propose that the charge transport in the arrays of FePt/CdSe nanocructures occurs via electron tunneling between metallic FePt cores rather than between 1Se states of CdSe shells. Similar model has been introduced by us to explain the charge transport in FePt/PbS nanostructures.11 Here we apply it to wider gap CdSe shell and explore the consequences of large difference in static dielectric constants of PbS (ε=175) and CdSe (ε=6.2).23 The electron transfer rate between sites A and B ( ΓA→B ) can be generally expressed as:24

{

(

ΓA→B ≈ g A g Bν 0 exp − 2 2m * ∆E / h 2

)

1/ 2

}

L


(1)

8

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where gA and gB are the numbers of occupied states in site A and unoccupied states in site B satisfying the conditions of resonant tunneling, ν0 is the pre-exponential factor known as “attempt-to-escape frequency”, m* is the electron effective mass, ∆E and L are the barrier height and width, respectively.

ΓA→B can be related to electron carrier mobility by using the Einstein relation25

µ e = ed 2 (6 k B Tτ ) , where e is the electron charge, d is the hopping distance, kB is the Boltzmann constant, T is the temperature, and τ is the hopping time inversely proportional to the electron transfer rate. It is usually accepted that the attempt-to-escape frequency ν0 is about the phonon frequency, typically ~1013 s−1. Metallic FePt cores provide much higher gA and gB compared to any quantum-confined semiconductor and 102 can be used as a conservative estimate for gAgB at the room temperature.11 Figure 1c shows ~8 nm distance of close approach between FePt cores of neighboring NCs. The use of In2Se42- MCC ligands allowed us to eliminate the insulating layers between NCs and we assumed L=8 nm and m*=0.13me26 for the tunneling barriers. For the reasons explained in the previous paragraph, it is difficult to come up with reliable estimate to ∆E. Instead, we used experimental value of µe and Eqs. (1) to obtain an estimate for ∆E~0.14 eV. We further suggest that the value of ∆E can be tuned by the gate voltage due to large Coulomb charging energy of small (3 nm) FePt cores. The charging energy of FePt cores can be estimated as Ec~e2/4πε0εrd, where ε0 is the permittivity of vacuum, εr is the dielectric constant of medium (CdSe in our case), d is the diameter of FePt core.1 In our case, Ec was as large as 77 meV. The gate coupled capacitively to the NC film through a 100 nm-thick SiO2 layer and resulted in accumulation 0.19e charge per volt applied between the source and gate electrodes. This charge can, at least partially, accumulate in FePt cores decreasing the ∆E by ~70 meV per


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each electron injected into the NC. In an idealized case of negligible charge trapping, VG = 30V can decrease of ∆E by over 400 meV, resulting in a ~2×105 increase of the channel current. We also studied the effect of magnetic field on charge transport through an array of FePt/CdSe

nanostructures.

The

all-inorganic

MCC-capped

magnet-in-semiconductor

nanostructures with the relatively high electrical conductivity can be a new-type model system to study magnetotransport at nanoscale.27 To study magnetotransport, the magnetoresistance (MR) in thin film of In2Se42- MCC-capped FePt/CdSe core-shell nanostructures were measured at low temperature. The resistance as a function of the external magnetic field (H) showed negative MR (the decrease of resistance under magnetic field) and the maximum MR value (Figure 4a) was ~7% at 5 K and 50 kOe. Because the MR of FePt/CdSe nanostructures was not saturated at 50 kOe field, the MR could probably be further increased at higher magnetic field. Also, the MR decreased with the increase of temperature and was very weak above 50 K. Among various suggested mechanisms to explain the MR behavior, the MR of FePt/CdSe nanostructures can be explained by the spin-dependent tunneling (SDT) mechanism.28 SDT has been generally observed in various NC systems such as Co29 and FePt/PbX.11 In SDT model, the Inoue et al predicted that the field dependences of MR in granular magnetic materials with a random orientation converge onto a single master curve in the plot of MR vs HT-1.30 The field-dependent MR of FePt/CdSe nanostructures measured at 5, 10, 20 K (Figure 4b) showed similar curves in the plot of MR vs HT-1 coordinates. Furthermore, the MR in SDT mechanism is suppressed by low-energy spin-flip scattering as temperature and electrical field increase. The MR of FePt/CdSe nanostructures also decreased with the increase of bias voltage (Figure 4c),29 which further supported the SDT mechanism of magnetotransport. The observation of SDT also


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supports our proposed charge transport mechanism based on the direct tunneling between metallic cores. In summary, we have showed that all-inorganic multifunctional nanostructures can exhibit high FET device performance and magnetotransport properties. The appropriate MCC ligands strengthened the electronic coupling between NCs, which enhanced the field effect mobility to 3.4 cm2/V·s in In2Se42- MCC-capped FePt/CdSe nanostructures. Furthermore, the MR of In2Se42MCC-capped FePt/CdSe nanostructures reached ~7% at 5K. These multifunctional properties of FePt/CdX (X=S and Se) nanostructures provided the possibility to design and prepare new type magnetic semiconductor nanostructures. Furthermore, we hope that better control of NC size, composition, and structure will further enhance the carrier mobility and magneto-response, providing new materials family for spintronic and data storage applications. For example, fctphase FePt/CdSe core-shell nanostructures could improve magneto-response due to high magneto-crystalline anisotropy.

Author Contributions ⊥These

authors contributed equally.

ACKNOWLEDGMENT The work was supported by University of Chicago NSF MRSEC Program under Award Number DMR-0213745. D.V.T. also thanks the Keck Foundation. This work was performed, in


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part, at the Center for Nanoscale Materials, a U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences User Facility under Contract No. DE-AC02-06CH11357

Supporting Information Available: Detailed experimental procedures and characterization methods. FT-IR spectra of nanostructures. Magnetic properties of nanostructures. Absorption spectra of nanostructure thin film. This material is available free of charge via the Internet http://pubs.acs.org.

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Figure 1. TEM images of as-synthesized (a) FePt/CdSe and (b) FePt/CdS, and In2Se42- MCCcapped (c) FePt/CdSe and (d) FePt/CdS core-shell nanostructures. Insets of (a) and (b) show high-resolution TEM images.


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Figure 2. (a) XRD patterns of FePt/CdSe nanostructures capped with a) original organic ligands, and b) In2Se42- MCC ligands before and c) after annealing at 200 oC for 1h, and FePt/CdS nanostructures capped with d) original organic ligands, and e) In2Se42- MCC ligands before and f) after annealing at 200 oC for 1h and g) FePt NCs used as core materials. Black vertical lines indicate X-ray diffraction patterns of bulk CdSe and CdS. (b) Uv-Vis absorption spectrum of FePt/CdSe and FePt/CdS nanostructures capped with the original organic ligands (blue line), and In2Se42- MCC ligands (red line).


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Figure 3. Gated transport of In2Se42- MCC-capped FePt/CdSe and FePt/CdS nanostructures. (a) Schematics of a field-effect transistor with a channel of FePt/CdSe and FePt/CdS nanostructure arrays. (b) Plots of drain current ID plotted versus drain-source voltage VDS, as a function of gate voltage VG and (c) plots of ID vs VG at constant VDS=2V used to calculate current modulation and linear-regime field-effect mobility for FET with n-channels of array of MCC-capped FePt/CdSe nanostructures. (d) Plots of ID plotted versus VDS, as a function of gate voltage VG and (e) Plots of ID and ID1/2 vs VG at constant VDS=30V used to calculate current modulation and saturationregime field-effect mobility for FET with n-channels of array of MCC-capped FePt/CdS nanostructures (f) Proposed alignment of the energy levels in In2Se42- MCC-capped FePt/CdSe nanostructures. Dashed lines correspond to charging states of the metallic FePt core.


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Figure 4. Magnetoresistance (MR) of In2Se42- MCC-capped FePt/CdSe nanostructures. (a) Lowbias MR measured at 5 (blue line), 10 (red line), and 20 K (orange line). (b) Plot of MR vs HT-1 at 5 (blue line), 10 (red line), and 20 K (orange line). (c) MR measured with applied voltage of 0.2 V (blue line) and 2 V (red line) at 5 K.


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Magnet-in-the-Semiconductor Nanomaterials: High Electron Mobility

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