Letter pubs.acs.org/JPCL
Multifunctional Doped Semiconductor Nanocrystals Santanu Jana, Bhupendra B. Srivastava, Somnath Jana, Riya Bose, and Narayan Pradhan* Department of Materials Science and Centre for Advance Materials, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032 S Supporting Information *
ABSTRACT: Multifunctional nanomaterials with combined magnetic and optical properties remain one of the most demanded materials in upcoming research. To obtain these materials, we report here several doped semiconductor nanocrystals that simultaneously show tunable emission in a visible and NIR spectral window, aboveroom-temperature ferromagnetism, and improved conductivity. These nanocrystals are designed by inserting Ni(II) as a dopant in various semiconducting hosts with binary, alloyed, and ternary composition, and the induced multifunctional properties are observed to be stable and reproducible. These semiconducting materials combined with fluorescence and magnetic properties would be useful for a wide range of applications spanning from life science to modern developing device technology.
SECTION: Plasmonics, Optical Materials, and Hard Matter
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semiconducting, optical, and magnetic properties with widewindow tunable emission and room-temperature ferromagnetism from a single domain. In this Letter, we have explored Ni2+ ion as a versatile dopant to induce both optical as well as magnetic center in various semiconductor host nanocrystals and to achieve the desired multifunctional nanocrystals. It is known that on doping, Ni2+ can generate ferromagnetism,22−24 but here, we have observed that Ni2+ can simultaneously be an optically active dopant to obtain intense, stable, and host band-gap-dependent tunable dopant emission. This has been observed in a wide variety of semiconductor hosts with binary, alloyed, and ternary compositions, and all show the tunable optical emission as well as room-temperature ferromagnetism. Moreover, the dopant emission that possesses several advantages, for example, minimized self-absorption and thermal, environmental, and photochemical stability, enables them to be a superior material, particularly for the applications in LEDs and biology. For selecting hosts, we preferred the semiconducting materials having visible and NIR optical absorption, which have wide scope for various applications. Under this category, ZnSe, alloyed CdZnS, and CuInS/ZnS (CIS) nancrystals are preferred, which have desired ZB crystal structures and also have the required tunable wide absorption window. Moreover, these wide varieties of hosts are selected for their suitability in achieving semiconducting, photovoltaic, and transport properties in addition to the magnetic and optical characteristics.
aterials exhibiting semiconducting and magnetic properties with room-temperature ferromagnetism are of great interest in spintronics.1,2 Materials showing semiconducting and optical properties with stable and tunable emissions are useful for optoelectronic devices.3−5 Similarly, materials having all of these combined semiconducting, magnetic, and optical properties can have additional applications in functional spintronics such as spin-optical devices (called spin-LED).6,7 Hence, multifunctional materials with such combined functionalities possess more degrees of freedoms for their versatile applications in recent developing technologies. Additionally, these materials are also extensively used in life science for imaging, diagnostics, drug delivery, and therapeutics.8−12 In recent years, much effort has been devoted to build these multifunctional materials, particularly in their nanoscale counterpart following colloidal synthetic methodologies. Several such materials are reported by coupling quantum dots with magnetic materials,8,12−16 designing core/shell materials with a magnetic core and fluorescent shell or vice versa,16 growing metal on semiconductor materials, and so forth.17,18 Apart from all of these developments, different doped semiconductor nanocrystals are also designed that show several combined functional properties and are also used as multifunctional materials in various applications.6,19,20 However, despite all of these successes, little progress has been made to design multifunctional semiconducting materials having above -room-temperature ferromagnetism. This remained a key issue for their practical applications in device and network technologies, particularly in spintronics.1,2 Recent developments also show that ferromagnetic materials can be an ideal contrasting agent in MRI.21 To our knowledge, there has been no such material reported till now that can have all © 2012 American Chemical Society
Received: August 1, 2012 Accepted: August 27, 2012 Published: August 27, 2012 2535
dx.doi.org/10.1021/jz3010877 | J. Phys. Chem. Lett. 2012, 3, 2535−2540
The Journal of Physical Chemistry Letters
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Figure 1. Representative photoluminescence (PL) spectra of Ni-doped ZnSe (a), CdZnS (b), and CIS (c). Samples were collected from the reaction flask at different time intervals. Corresponding UV−visible spectra are provided in the Supporting Information (Figure S1). Excitation wavelength for ZnSe, CdZnS, and CIS are 350, 365, and 400 nm, respectively. The high-energy peak in the first spectra of the Ni-doped ZnSe sample is from undoped ZnSe. The quantum yield for CIS increases up to 35% after ZnS shelling. Insets are digital pictures under 365 nm UV excitation. (d−f) Magnetization versus magnetic field for all three doped semiconductor nanocrystals. Insets are coercivities of respective nanocrystals. (g−i) Voltagedependent current of all three doped and undoped samples under white light irradiation, and the inset in (h) is a thin film made by precipitating Nidoped CdZnS nanocrystals magnetically on a glass slide. The SEM picture of the slide is shown in the Supporting Information (Figure S2).
Figure 2. (a−c) TEM images of Ni-doped ZnSe, CdZnS, and CuInS2, respectively and (d) HRTEM of Ni-doped CdZnS. (e) EDAX and (f,g) HAADF-STEM images and the elemental mapping for S, Zn, Cd, and Ni, respectively. (h) Representative XRD of Ni-doped ZnSe, CdZnS, and CIS.
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dx.doi.org/10.1021/jz3010877 | J. Phys. Chem. Lett. 2012, 3, 2535−2540
The Journal of Physical Chemistry Letters
Letter
Synthesis of these doped nanocrystals has been carried out following the growth doping strategy. Host nanocrystals are synthesized following different reported literature methods.25−28 Doping is carried out by decoupling the nucleation and growth processes. Soon after the nucleation, the growth is ceased by lowering the temperature and required amount of dopant (Ni-stearate) precursor is introduced. The reaction is kept at the same temperature for a few minutes (typically 10− 30 min) to allow the dopants to adsorb onto the surface of the host nanocrystals. Then, the growth is resumed to get the desired doped nanocrystals. Details of the synthesis and doping methodologies are provided in the Supporting Information. The concentration of nickel was maintained at
