Spin-Polarized Scanning Tunneling Spectroscopy of Diluted Magnetic

Oct 17, 2014 - C , 2014, 118 (44), pp 25786–25791 ... From the SP-STS, we have calculated normalized density of states (NDOS) of the diluted magneti...
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Spin-Polarized Scanning Tunneling Spectroscopy of Diluted Magnetic Semiconductor Quantum Dots Sudipto Chakrabarti and Amlan J. Pal* Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India S Supporting Information *

ABSTRACT: We have formed ultrathin films of cobalt-doped ZnO quantum dots (QDs), aligned their magnetic domains, and recorded spin-polarized scanning tunneling spectroscopy (SP-STS) with a nickel tip whose magnetization vector was also aligned. From the SP-STS, we have calculated normalized density of states (NDOS) of the diluted magnetic semiconductor (DMS) QDs. We have observed that the intensity of peaks in NDOS spectra has depended on the mutual orientation of the magnetization vectors of the tip of the scanning tunneling microscope (STM) and the domains of the QDs. A higher intensity in the NDOS spectrum is observed in tip−QD systems having their domains aligned parallel to each other as compared to their antiparallel configuration. We have performed a range of control experiments to validate the observation. We hence infer that the orientation of the magnetization vector or the magnetic state of ferromagnetic QDs relative to that of the tip of a STM can be read or probed by recording SP-STS.

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INTRODUCTION Scanning tunneling spectroscopy (STS) has been a powerful technique to study lower dimensional systems1−7 and single organic molecules.8 The lower dimensional systems that have been considered in this direction ranged from two-dimensional graphene1,2 or graphene-like sheets3 to zero-dimensional quantum dots of semiconductors and metals.4−7 With a noble metal as a tip of STS, tunneling of electrons to or from the nanostructure is studied to obtain the energy levels and density of states of the materials in isolation.9 Energy levels at an interface between two materials of complex systems, such as organic−inorganic hybrids,10,11 pn-junction rectifier in a nanorod,12 or cross-section of bulk-heterojunction solar cells,13 have also been mapped with the STS. In spin-polarized scanning tunneling spectroscopy (SP-STS), where spin degrees of freedom of electrons are also added during the tunneling process, a ferromagnetic (FM) tip is used to characterize nonmagnetic materials on suitable magnetic substrates.14−16 Magnetization vectors of the tip and the substrate are aligned either parallel or antiparallel to each other leading to different resistive states or magnetoresistance of the material of interest. Ferromagnetic quantum dots (FM-QDs) on nonmagnetic substrates have also been imaged and characterized with a spinpolarized scanning tunneling microscope (SP-STM).17,18 Materials for FM-QDs in such studies were restricted to ferromagnetic cobalt nanoparticles. Other ferromagnetic materials that have been characterized with a conventional STM tip were suitably doped ZnO and Fe3O4 nanoparticles.19,20 In tip/FM-QD/substrate systems, the spinpolarized tunneling current, or rather the normalized density of states (NDOS), should contain the signature of the mutual alignment of the magnetization vectors of the tip and the QDs. © 2014 American Chemical Society

In this work, we envisioned employing the FM nature of the SP-STM tip to study the domain of a diluted magnetic semiconductor (DMS) quantum crystal in isolation. That is, we aimed to know if the NDOS determined from SP-STS would depend on the mutual alignment of magnetization vectors of the tip and the domain of a QD. In such a case, the magnetic state of the domain of the QD (or the bit per quantum dot) can be read by recording SP-STS. The DMS that we studied in this work is ferromagnetic Co2+-doped ZnO (Co@ZnO).21

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EXPERIMENTAL METHODS Growth of Co@ZnO Nanocrystals. Co@ZnO nanocrystals were grown following the colloidal synthetic route established by Gamelin and his research group.22 In the synthesis process, which ensures Co2+ to be isotropically doped at Zn2+ sites throughout the ZnO nanocrystals, precursor salts of zinc and cobalt, namely, zinc acetate dihydrate Zn(CH3COO)2·2H2O and cobalt(II) acetate tetrahydrate Co(CH3COO)2·4H2O, were dissolved in 15 mL of dimethyl sulfoxide (DMSO). While the concentrations of Zn2+ and Co2+ ions were selected in order to achieve a particular doping concentration of cobalt, the total concentration of the metal ions in the solution was 0.1 M. To grow the doped nanocrystals, the solution was stirred vigorously for 15 min at room temperature before 5 mL of 2.5 mM tetramethylammonium hydroxide (TMAH) solution in dry ethanol was added dropwise at a rate of 2 mL/min. The color of the reactants changed from pink to blue indicating formation of Co@ZnO nanocrystals. After the addition of TMAH, the stirring process Received: August 25, 2014 Revised: October 17, 2014 Published: October 17, 2014 25786

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For SP-STS, magnetically aligned nickel tips and ultrathin films of Co@ZnO having their domains oriented were used. The ultrathin films of QDs and the nickel tip were placed in the STM chamber in such a manner that the direction of magnetization vectors of the tip and the QDs were oriented mutually parallel or antiparallel to each other. We imaged two films with a single tip to compare NDOS of QDs in two tip− QD configurations. Imaging of the quantum dots and measurement of scanning tunneling current were carried out on many different points on each ultrathin film to bring out reproducibility of the results. During approach of the tip, a preset current was achieved at 2.0 V through a feedback loop of the STM controller. To record tunneling current versus tip voltage (I−V) characteristics for the determination of NDOS, the tip voltage was swept between −2.5 and 2.5 V in both directions.

was continued for some more minutes to stabilize the nanocrystals. They were separated by precipitating them with an addition of 40 mL of ethyl acetate. After removal of the supernatant by centrifugation, the nanocrystals were isolated and redispersed in ethanol or DMSO for further use. The content of cobalt with respect to zinc in the doped ZnO nanocrystals was selected to be up to 6%. Pristine or undoped ZnO nanocrystals were also grown to carry out control experiments. Characterization of the Nanocrystals. The range of nanocrystals was characterized with optical absorption spectroscopy, X-ray diffraction (XRD) patterns, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), and energydispersive X-ray (EDX) analysis. Film Formation of the Nanocrystals. Since we would be probing domains of the ferromagnetic nanocrystals in isolation, we formed ultrathin films of the nanostructures on arsenicdoped silicon ⟨111⟩ wafers, which had a resistivity of 5−10 mΩ·cm, acting as an electrode. The wafers were cleaned and treated with dilute hydrofluoric acid in order to remove the native oxide on the surface. An ultrathin layer of Co@ZnO nanocrystals was spun on the silicon substrates at 3800 rpm. The speed of rotation was chosen in such a manner that the thickness of the film matched the diameter of the nanocrystals. To select the desired speed of rotation, we first formed thin films at different rotating speeds; we then measured the thickness of the films by recording atomic force microscopy images of a depth profile of an intentional scratch on the films. From a plot of thickness versus speed of rotation (Figure S1 in the Supporting Information), we selected a speed that would ensure formation of an ultrathin film having a thickness matching the diameter of Co@ZnO QDs or nearly one monolayer of the nanocrystals on Si surfaces. In order to orient the domains of Co@ZnO QDs, the ultrathin films were placed in an external magnetic field of 330 mT. Such a field was enough to achieve magnetic saturation in the QDs as evidenced from the literature.20,22 Direction of the external field was such that the magnetization vectors of the DMS QDs become aligned parallel to the plane of the substrate facing one particular direction. After alignment of magnetic moments, the ultrathin films of the QDs were placed in a loadlock chamber of a STM before finally transferring them to the main chamber of the microscope. Spin-Polarized Scanning Tunneling Spectroscopy (SPSTS). An ultrapure nickel (99.9%) wire having a diameter of 0.25 mm was used for SP-STS measurements. They were first cleaned and etched with a tip-etching system (Unisoku UTE1001). The parameters for etching were set in such a manner that the radius of curvature of the tips became less than 20 nm, at which the magnetization vector at the apex should be aligned perpendicular to the long axis of the tip.23 The magnetization vector at the apex was further reinforced by placing the tip in an external magnetic field. The tips were then inserted to the STM chamber following the usual procedure. For conventional STS, platinum/iridium (Pt/Ir, 80%/20%) wires that were mechanically cut to form a tip were used. STS measurements were carried out with a PAN-style UltraHigh Vacuum STS (UHV-STS) manufactured by M/s RHK Technologies, USA. While the base pressure of the microscope chamber was 4.0 × 10−10 Torr, the temperature of both the substrate and the tip was 80 K.

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RESULTS AND DISCUSSION Characterization of Co@ZnO Nanocrystals. The nanocrystals were characterized with optical absorption spectroscopy, XRD patterns, XPS, TEM and HR-TEM, and EDX analysis. Optical absorption spectra of different Co@ZnO nanocrystals in dispersed solution, as presented in Figure 1, show a band in

Figure 1. Optical absorption spectra of different cobalt-doped ZnO quantum dots in dispersed solution. Inset shows the longer wavelength section of dispersed solutions having a higher concentration.

the near-UV region. With cobalt doping, the band shifted toward the shorter wavelength region. The shift is related to a decrease in the diameter since with an incorporation of cobalt in ZnO crystal the dopants inhibit nucleation and growth of nanocrystals leading to a blue-shift in the optical absorption band of the doped quantum dots.22 With cobalt doping, new bands appeared in the visible region with peaks at around 566, 606, and 648 nm. It has been inferred that ligand field absorption resulted in formation of the bands that represented transitions from 4A2(F) to 2A1(G), to 4T1(P), and to 2T1(G), respectively.22,24 These transitions arose due to tetrahedral coordination of Co2+ ions substituted at the Zn2+ sites in the wurtzite structure of ZnO nanocrystals. Incorporation of cobalt as dopant could also be observed from EDX analysis. Elemental composition of the nanomaterials for different doping concentrations has been summed up in Table S1 in the Supporting Information. The results show that the content of cobalt in the doped quantum dots increased with an increase in the amount of cobalt acetate added during the growth process. The crystalline nature of the quantum dots was confirmed from XRD analyses. Figure S2 in the Supporting 25787

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assigned to the adsorbed oxygen in the sample.25 The XPS spectrum of Co 2p could be resolved into 2p3/2 and 2p1/2 states appearing at 782.2 and 797.8 eV, respectively, that match the energies of Co2+ states supporting the presence of Co2+ at the Zn2+ sites in forming doped oxide semiconductors. Tunneling Current and NDOS: Effect of Relative Alignment of Magnetization Vectors of the Tip and the FM-QDs. We have recorded spin-polarized scanning tunneling current in ultrathin films of the ferromagnetic QDs with a nickel tip. In Co@ZnO nanocrystals, ferromagnetism appears in clustered QDs through formation of a single magnetic domain. We recorded SP-STS with magnetization vectors of the tip and of the FM-QD being both aligned. The relative alignment of the magnetization vectors was either parallel or antiparallel. The spin-polarized tunneling current as a function of the tip voltage in the antiparallel configuration is presented in Figure 3. The I−V characteristics were recorded at many different points on each thin film.

Information shows XRD spectra of undoped and doped ZnO nanocrystals. All the spectra readily matched the wurtzite structure of the nanocrystals (JCPDS file #36-1451). Upon cobalt doping in ZnO, bands related to other subphases did not appear in the spectra. Figure S3 in the Supporting Information shows a typical TEM image of doped ZnO nanocrystals. The image shows that the nanocrystals were mostly spherical and monodispersed. A HR-TEM image, as presented in the inset of the figure, shows a lattice spacing of 0.26 nm which matches with the ⟨002⟩ planes of the wurtzite structure. To know the level of monodispersivity of the quantum dots, we have added the diameter distribution in the form of a histogram in the figure. The distribution shows a shift in diameter of the nanocrystals upon cobalt doping. We have carried out XPS studies to obtain information about the valence state and elemental compositions of Co@ZnO nanocrystals. A full scan spectrum of a typical case (6% doped), as presented in Figure 2, shows peaks corresponding to Zn 2p,

Figure 3. Current−voltage characteristics of an ultrathin film of cobaltdoped ZnO quantum dots with a nickel tip. Magnetization vector of the domains of the QDs and the nickel tip both were aligned antiparallel to each other. The tip-approaching condition was 0.3 nA at 2.0 V. The temperature of the tip−QD system was 80 K. Results obtained from 15 different points on the films are shown in the figure with the thick line being the average of the characteristics. A couple of surface topographies of an ultrathin film are shown in the inset of the figure.

We have recorded STS of the QDs for both parallel and antiparallel configurations of the tip−QD system at different tip-approaching currents. A couple of typical surface topographies of an ultrathin film are shown in the inset of Figure 3. Schematic representations of the tip−QD systems during the measurement are shown in Figure 4(a) where orientation of the tip is made to a specific direction. In STS, since the system resembles a double-barrier tunnel junction (DBTJ) involving the tip−QD and QD−substrate tunnel barriers, the tunneling current always passes through the set point, which was used during the approach of the tip. Therefore, the magnitude of tunneling current will not depend on the QDs since the tip− QD height is adjusted to attain the set point during approach of the tip. We therefore carried out the measurements at different tip−QD distances by using different set points for the approach. From the I−V characteristics of the two configurations, we therefore have calculated the NDOS of the QDs under the two probing conditions (Figure 4b). Since the differential tunneling conductance is proportional to the sample local density of states, a spectrum of NDOS has been developed from the

Figure 2. Full-range XPS spectrum of cobalt-doped ZnO quantum dots. High-resolution XPS spectra of C 1s, Zn 2p, Zn 3s, O 1s, and Co 2p of the material are shown in (b)−(f) in sequence.

Zn 3s, O 1s, and Co 2p states confirming the occurrence of the elements in the nanomaterial. The high-resolution spectra of the peaks corresponding to the elements are presented in the figure. Here the binding energies of the peaks have been corrected with reference to C 1s peak at 284.5 eV. As can be observed in the figures, the spectrum corresponding to Zn 2p was resolved into 2p3/2 and 2p1/2 peaks appearing at 1022.0 and 1045.0 eV, respectively. The Zn 2p peaks, along with the Zn 3s peak at 140.2 eV, inferred that the metal was in a doubly oxidized state in the nanocrystals. The spectrum of the O 1s level of oxygen should consist of a single core level peak at 531.1 eV corresponding to the O2− ions in the wurtzite structure of ZnO. An associated shoulder at 532.6 eV has been 25788

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vectors and was higher when the vectors of the tip and the QDs were parallel as compared to that when they were antiparallel to each other. In other words, spin-polarized electrons could “see” a larger number of available states at which polarized electrons could be injected when the magnetization vectors of the tip and the QDs were parallel. When they were antiparallel, the tip found fewer states in the QDs to inject spin-polarized electrons. The NDOS spectra in addition can provide the energy of conduction and valence band edges. Since the bias was applied with respect to the tip, the peaks at the positive voltage region, at which electrons could be injected to the QDs, denoted the location of conduction bands. Similarly, the peaks at negative voltages denoted the valence band at which electrons could be withdrawn from the QDs. We find that the conduction band of Co@ZnO was located closer to the Fermi energy which was fixed at 0 V and could be matched to the work function of the substrate electrode. The NDOS spectra are hence in agreement with the n-type nature of the material. The position of the band edges and also the intensity of the bands in NDOS spectra, which were calculated from I−V characteristics recorded at many different points on the ultrathin films, are shown in Figure 4(c). The intensity of the available states for electron injection was higher when magnetization vectors of the tip and the QDs were parallel to each other as compared to that when they were antiparallel. The energy of the conduction and valence band edges and their difference or the transport gap of the QDs expectedly did not depend on the configuration of magnetization vectors of tip−QD systems since the same material was being probed in the two cases. The value of the transport gap, which turned out to be 3.01 eV, matches reasonably well with the reported results. The magnetization vector of the QDs has hence played a role during the tunneling of (spin-polarized) electrons. When they were antiparallel, the QDs contributed to a higher resistance in the double barrier tunnel junction (DBTJ) for the injection of spin-polarized electrons. In other words, the direction of magnetization vector of the QD with respect to that of the tunneling electrons controlled a component of tunneling resistance causing QDs’ magnetic state to act as a parameter that could be read or probed.

Figure 4. (a) Schematic representations of the tip−QD systems in the two measurement conditions. (b) Normalized density of states (NDOS) of an ultrathin film of Co@ZnO QDs with a nickel tip, as calculated from I−V characteristics. Magnetization vectors of the domains of the nickel tip and the QDs were aligned either antiparallel or parallel. Vertical broken lines in the positive and in the negative voltage represent the location of conduction and valence band edges, respectively, of the QDs. (c) Intensity of the peaks corresponding to the valence and conduction band edges of the two tip−QD configurations. Values obtained from the NDOS spectra, as calculated from I−V characteristics recorded at 15 different points on the films, are shown in the figure.

numerical derivatives (dI/dV)/(I/V) of the average I−V characteristics at each point. The plot shows that the NDOS depended on the mutual alignment of the magnetization

Figure 5. (a) Co@ZnO QDs with a Pt/Ir tip and (b) undoped ZnO QDs with a magnetized Ni tip. Schematic representation of the tip−QD system in the two measurement conditions and NDOS of the QDs of the two tip−QD configurations are shown in each of the figures. Black and red lines represent the tip−QD system as shown in the left and the right side of the schematic representation, respectively, in each of the plots. 25789

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The Journal of Physical Chemistry C We have carried out the same set of experiments where the orientation of the magnetization vector of the nickel tip was made opposite to that shown in Figure 4(a). Schematic representation of the tip−QD systems during the measurements, NDOS of the QDs under the two probing conditions (as evaluated from the I−V characteristics), and the position and intensity of the band edges in NDOS spectra as calculated from I−V characteristics recorded at many different points on the films are shown in Figure S4 in the Supporting Information. Here also, the NDOS was higher when the magnetization vectors of the tip and the QDs were parallel as compared to that when they were antiparallel to each other. Control Experiments. To evidence further that the spinpolarized electrons could “see” a larger number of available states in the QDs when the magnetization vectors of the tip and the QDs were parallel, we have carried out a range of control experiments. First of all, all the measurements were repeated with a paramagnetic Pt/Ir tip. NDOS of the two ultrathin films, which have domains (of QDs) oriented and were earlier characterized with a magnetically aligned nickel tip, returned indistinguishable characteristics. The intensity of the bands did not differ in the two ultrathin films of Co@ZnO. Measurements with an unaligned nickel tip retuned similar results. We also recorded STS of undoped ZnO with an aligned nickel tip. In this case also, the NDOS spectra did not differ for the two orientations of the magnetization vector of the nickel tip. NDOS spectra along with the schematic representation of tip− QD configurations are shown in Figures 5(a) and (b). The indistinguishable NDOS in both the experiments appeared since one of the magnetization vectors of tip−QD systems was absent in the two cases. The control experiments hence infer that the spin-polarized electrons could read the state of the domain of ferromagnetic QDs.

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CONCLUSIONS In conclusion, we have characterized an ultrathin layer of ferromagnetic Co@ZnO QDs with a nickel tip of scanning tunneling microscope at 80 K. Spin-polarized scanning tunneling spectroscopy of the QDs was recorded with the magnetization vectors of domains of the tip and of the QDs aligned either parallel or antiparallel to each other. With the spin degrees of freedom of the electrons operative, we have observed that the NDOS of the QDs depended on the mutual alignment of the vectors and was higher when the tunneling electrons’ spin was in unison with the magnetization vector of the QDs as compared to that when the vectors were antiparallel to each other. Through a range of control experiments, we have inferred that the relative orientation of the magnetization vector of QDs could be read or probed from tunneling spectroscopy of spin-polarized of electrons. ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S4 and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

The authors acknowledge financial support through Nano Mission projects.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-33-24734971. Fax: +91-33-24732805. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 25790

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