ARTICLE pubs.acs.org/crystal
Chromium-Doped Germanium Nanotowers: Growth Mechanism and Room Temperature Ferromagnetism Amar S. Katkar, Yen-Chang Chu, Li-Wei Chu, and Lih-Juann Chen* Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, Republic of China
bS Supporting Information ABSTRACT: The growth of high density germanium (Ge) nanostructures (nanowires and nanotowers) and influence of chromium (Cr) doping on its morphology have been investigated. It is observed that in the absence of Cr, Ge nanowires (NWs) are formed whereas the presence of Cr leads to the growth of Ge nanotowers (NTrs), wherein all other experimental parameters were kept constant. Independent of the Cr concentration, the crystal structures of both nanostructures, that is, NWs and NTrs, are identical. Traditional vaporliquidsolid (VLS) process was used to explain the growth mechanism of the top of the Ge NTr, consisting of a thin nanowire with a gold tip. The bottom of the NTr consists of lateral (111) and (001) planes of polygonal structure; VLS process along with vapor-solid (VS) process accounts for its further growth. The mechanism for the dopant-dependent morphological changes in Ge nanostructures is clarified. The magnetic properties of the NTrs were probed using a superconducting quantum interference device (SQUID). Room-temperature ferromagnetism in Cr-doped Ge nanotowers was discovered. These findings can thus be exploited for controlling the morphology and generating the magnetic ordering in nonmagnetic semiconductors to develop stabilized novel spintronics devices.
’ INTRODUCTION Ge has regained significant interest in the research community due to its high carrier mobility, making it as a candidate material for high speed nanoelectronics applications.1,2 The controlled synthesis of anisotropic semiconductor nanostructures is promising for the next generation electronics and photonics devices.3 Moreover, various Ge nanostructures like nanowires,410 nanospikes,11 nanorods,12 nanotubes,13 nanoislands,14 and nanodots,15,16 have been synthesized successfully. The morphological changes in nanostructures are well-known contributors for tuning their properties, which are being investigated intensively. The influence of shape and morphology of different nanostructures enhanced the broad range applications, such as nanowires for FETs,1719 high capacity Li ion battery anodes,20 two-terminal nanoelectromechanical devices,21,22 photoconductors,23 sensors,24 nanorods for optoelectronics,12 nanodots for solar batteries and thermophotovoltaics, quantum dot-based lasers.15,16 Nanostructures like nanotowers (NTrs) are of growing interest, because of their properties such as photoluminescence,2528 superhydrophobic performance29 field emission,30 and ferromagnetism.31 Successful synthesis of Ntr structures of In2O3,32,33 Cr-doped In2O3δ,31 ZnO,28,30,3436 ZnMgO,27 Ga-assisted ZnO,26 stearic acid modified ZnO29 and ZnS25 have been reported before. According to our knowledge this is the first report wherein we have reported the synthesis of Ge nanotowers initiated by thin nanowires with a gold tip. In this investigation, two kinds of Ge nanostructures (Ge NWs and Cr-doped Ge NTrs) were grown on silicon wafer via a vapor r 2011 American Chemical Society
transport method. The morphological and structural features of both the nanostructures have been investigated to clarify the role of Cr source. To demonstrate the growth mechanism of Cr-doped Ge Ntrs, three experiments have been performed at different reaction times using similar experimental conditions used for nanotowers. Vapor liquidsolid (VLS) growth mechanism accounts for the growth of NTrs which are initiated by thin nanowires with gold catalyst. The lateral growth of polygonal facets (bottom of the NTr) followed by simultaneous longitudinal (1D) growth below the critical length (LC) has been explained by vapor-solid (VS) mechanism along with the VLS mechanism. The Cr atoms may be remarkably effective on determining diffusion length of the Ge adatoms and could be responsible for the formation of NTr structure. Moreover Cr dopant can also play vital and complementary role to generate the magnetic ordering at high temperature in Ge matrix.37 In this work, the room temperature ferromagnetism was observed in Cr-doped Ge NTrs. Thus GeCr complex nanostructures, that is, NTrs will be very promising materials for future devices and can open a new door to fabricate Ge based dilute magnetic semiconductor (DMS) materials as a hard magnet for high temperature spintronics devices.
’ EXPERIMENTAL SECTION P-type silicon (001) wafer was ultrasonically cleaned in ethanol for 10 min and 2-nm-thick gold film was deposited onto it at room temperature Received: February 4, 2011 Revised: May 15, 2011 Published: May 17, 2011 2957
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Figure 1. (a, b) FESEM images (low and high magnifications) of Ge NWs. (c, d, and insets) FESEM images (low and high magnifications) of Cr-doped Ge NTrs. Scale bar of right side and left side FESEM images are 10 and 1 μm, respectively. Insets show magnified FESEM images of top and bottom part of single NTr with scale bar of 500 nm. in an electron beam evaporation system at a pressure of ∼5 106 Torr. The Ge NWs and Cr-doped Ge NTrs were grown by a vapor transport method using a three zone furnace. A mixture of commercial Ge (99.999%, Alfa Aesar) and GeO2 (99.999%, Alfa Aesar) was used to synthesize Ge nanowires with a 1:3 ratio. Ge, GeO2, and CrO3 (99.99%, Alfa Aesar) powders were used to synthesize Cr-doped Ge NTrs with a 3:9:1 ratio and were placed in an alumina boat, which was heated to a peak temperature of 1100 C in zone-I. The 2 nm gold coated silicon wafers were placed in zone-II and zone-III at 600 C. The samples were heated to 1100 C at a rate of 20 C/min with the reaction time of 60 min and with a 60 sccm Ar flowing through the tube. After completion of reaction, the furnace was allowed to cool to room temperature. Black and brown products were obtained in case of NWs and NTrs, respectively, on the silicon wafer. The reaction of Cr-doped Ge NTrs was repeated for different reaction times (15, 30, and 45 min). After the growth process, resulting products were collected and used to determine their crystalline phase using X-ray diffraction (XRD) (Shimadzu). Morphological studies of the products have been performed with a field emission scanning electron microscope (FESEM) (JSM-6500F) and JEOL-2010 transmission electron microscope (TEM) (200 kV), equipped with an energy-dispersive spectrometer (EDS).
’ RESULTS AND DISCUSSION Figure 1a shows the low magnification FESEM image of a high density of Ge NWs, grown uniformly on the Si (001) substrate. The diameters and lengths of the NWs are in the range of 60100 nm and 12 μm, respectively. Figure 1b depicts a magnified FESEM image of a selected area in Figure 1a, which shows Ge NWs with catalytic gold at the tip. As the gold catalyst exists at the tips of the NWs, traditional VLS mechanism accounts for this catalytic growth mechanism of Ge NWs.38 Figure 1c shows a high density of Ge NTrs meshed up with each
Figure 2. XRD spectra of obtained NWs and NTrs.
other which looks like a network of NTrs. As shown in the high magnification FESEM image and its inset of the top and bottom parts of a single NTr (Figure 1d), these NTrs consisted of two parts; a thin nanowire at the top and a periodic polygonal-column structure with large diameter at the bottom revealing the NTr-like structure. The diameter and length of the top part (thin nanowire) of the NTrs were in the range of 60100 and 400 nm -2 μm respectively, similar to the Ge NWs described previously. Catalytic gold particles were also observed at the tips of the NTrs. The stems of the NTrs consist of a polygonal-column structure with stacking facets. The size of the polygons ascends from the top and remains almost constant. Thus most of the stem part, that is, the bottom of the NTr, is composed of truncated polygons with relatively uniform size of about 300500 nm. As shown in 2958
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Figure 3. (a) TEM image of Ge NTr. (b, c) EDS spectra of top (nanowire) and bottom (polygonal column) of the Ge NTr respectively. (d) Magnified TEM image of nanowire part and (e) its HRTEM image and inset is SAED pattern.
Figure 1ad, the morphology of Ge NWs has been changed to NTrs, after introduction of Cr source (CrO3 powder) together with Ge source (Ge and GeO2 powders). Figure 2a and 2b depict the XRD spectra of the obtained products of Ge NWs and Ge NTrs, respectively. Both diffraction peaks could be indexed to the cubic Ge (JCPDS Card 89-2768) and GeO2 (JCPDS Card 23-0999) crystal structure. Careful examination of both XRD peaks revealed that the crystal structures of both nanostructures are the same. The strong GeO2 peaks observed in XRD is likely due to crystalline shell of NTrs (Supporting Information Figure S2) and crystalline GeO2 particles on the substrate (The FESEM images (Figure 1 c,d) of Crdoped Ge NTrs show that some particles are observed at the bottom of NTrs on the substrate, which are conjectured to be crystalline GeO2 particles). As described earlier, the only different parameter in the growth of Ge NWs and Ge NTrs was the use of Cr source in the case of growing NTrs. From the FESEM images and XRD peaks, for both the samples (NW and NTr), it is apparent that Cr is responsible for changing the morphology. However, no change in the crystal structure is noted. Cr might have contributed to change the conditions of reaction and indirectly to change the growth conditions, which results in the formation of NTrs. The influence of doping impurities on the morphology of nanostructures39,40 suggests that, a small amount of doping of transition element such as Cr can play a significant role in the morphological change of semiconductor nanostructures and indirectly can tune their properties. The detailed characterization of NTrs was performed by TEM analysis. Figure 3a depicts a TEM image of an Au tipped Ge NTr with a nanowire diameter of about 80 nm, wherein the diameter of the stem increases from about 100 to 500 nm. A thin oxide layer is also seen in the TEM image. The oxide layer is thicker on the stem, that is, at the periphery of the polygonal-column as compared with the side wall of a nanowire at the top. The TEM analysis of the shell of stem part of Ge NTr was also carried out. The shell of stem of the Ge NTrs was found to consist of crystalline GeO2 (Supporting Information Figure S2). The EDS spectra (Figure 3b and 3c) obtained from the top, that is, the nanowire and the stem confirmed the direct evidence of the presence of Cr throughout the NTr. To determine quantitatively of the Cr concentration in Cr-doped Ge NTrs, XPS multiple scan of Cr 2p for Cr-doped Ge NTrs was carried out (Supporting Information Figure S1), which shows Cr 2p3/2 and 2p1/2 peaks.
The binding energies for Cr 2p3/2 and Cr 2p1/2 peaks are 578 and 588 eV respectively.41a,b The atomic % of Cr is 16.5. Magnified image and HRTEM image of the peripheral part of the nanowire and the corresponding SAED pattern are shown in Figure 3d, 3e and its inset, respectively. The HRTEM image and SAED pattern reveal that Cr-doped Ge NTr is single crystalline and the growth direction of the NTr is [111]. In addition, phase related to Cr was not detected. The growth of Ge NTRs in Æ111æ directions coupled with the cross-like appearance in the top view SEM image shown in Figure 3c indicates that the Ge NTrs were grown epitaxially on Si[001] substrate. In order to clarify the growth mechanism of Cr-doped Ge NTrs in details, we carried out three sets of experiments with different reaction times (15, 30, and 45 min), wherein all other experimental parameters were kept the same. Figure 4a shows FESEM image of a high density of Ge NWs grown with a reaction time of 15 min. The diameters of NWs structures are in the range of 60100 nm, with a catalytic gold particle observed at the tip. At 30 min of reaction time, a mesh of nanobottle-like structures were obtained, which is shown in Figure 4b. It reveals that, the lateral growth of the polygons is initiated after the formation of nanowire. As shown in the schematic next to the FESEM image of Figure 4b, the specific length of the nanowire above where the lateral growth has initiated was marked as a critical length (LC). From Figure 4b we can also observe that, the different NTrs have different critical lengths. The amount of concentration or distribution of Cr in the nanowires may determine the critical length (LC) of NTrs and that is why we can observe nanobottles with the variation in critical lengths of nanowires, that is, each individual nanobottle has its own critical length depending upon the concentration and distribution of Cr throughout. As the diameter of Cr (III) ion (0.63 nm) and Ge (IV) ion (0.53 nm) is close enough, the substitution of Cr in Ge lattice is most likely possible in the case of Ge NTr. Similarly, for 45 min of reaction time, further growth of the stem is observed to form a tower like structure. The FESEM image (Figure 4c) reveals the growth of Ge NTrs, but the length of the NTr was shorter than that for the reaction time of 1 h. (Figure 1d and 1e). Next to the Figure 4ac, individual FESEM images of nanostructures at different reaction times with their respective schematics have been shown. The FESEM images show growth of nanowires at 15 min, nanobottles (from where stacking of polygons initiated) at 30 2959
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Figure 4. FESEM images and its schematics with individual nanostructures for reaction times of (a) 15, (b) 30, and (c) 45 min and (d) individual FESEM images of nanostructures at respective reaction times with its schematics. Scale bar is 1 μm for all FESEM images.
min and NTr (further continuous growth of stacking) at 45 min of reaction times.
’ GROWTH MECHANISM According to the growth sequence observed in Figure 4, we can conjecture about the growth mechanism of Cr-doped Ge NTr and propose the growth sequence as shown in Figure 5. At the initial stage of growth, the gold particle acts as a catalyst for the nanowire (at the top part of the nanotowers) grown by VLS process with the same growth mechanism as that of the Ge NWs. After the growth of nanowire structure reaches a critical length (Lc), the stack-layer structure starts to grow. The lateral growth of stack-layer structure below the critical length (LC) is explained using VS growth process along with the VLS process. VS process can be effective to induce
anisotropic growth of a crystal that has low index facets31 along with the VLS process. As shown in Figure 5A, initially three possible ways of Ge adatom movements were expected: (1) diffusion of adatoms from the substrate and along the wall of the nanowire to the droplet, (2) desorption of adatoms, that is, release into the space, and (3) adatoms incorporation into the substrate. Some of Ge atoms from the vapor phase diffused into the catalyst. But as the catalyst acts as a sink for the adatoms, the concentration of the Ge atoms diffuse on the side wall near the catalyst will be the lowest and it will increase on the side wall away from the catalyst. As impurities can decrease the diffusion length of other adatoms,42 Cr atoms may decrease the diffusion length of the Ge adatoms. Once the concentration of Ge reaches a critical value, the growth process on the wall of nanowire above the critical length (LC) is initiated and the lateral growth of facets occur as shown in 2960
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Figure 5. Schematic illustration of step by step growth of Cr-doped Ge NTrs.
Figure 5B (blue polygon), that is, the growth of first polygon takes place. As the deposition rate of Ge atoms is constant, the concentration distribution of Ge adatoms at the catalyst droplet will be constant during the 1D growth by VLS and therefore the critical length (LC) above which lateral growth starts will be constant during complete growth of a nanotower. The facets of polygon are (111) and (001) planes because the (111) and (001) planes of Ge are known to have lower surface energy than its other {hkl} planes and they are energetically preferable.39 After the 1D growth of nanowire through the catalyst (Figure 5C, pink block below catalyst), above the critical length (LC), new polygon will form (Figure 5C, pink polygon). At the same time some adatoms will diffuse onto the first (Figure 5C, blue polygon) polygon at the bottom, so that it will increase its size (Figure 5C, pink over blue color polygon). As shown in Figure 5D, again 1D longitudinal growth by VLS through catalyst will occur (yellow block) and above the critical length new polygon (yellow color polygon) will grow. At the same time there will be increase in the size of first polygon at the bottom (Figure 5D, yellow over pink f blue polygon) and second polygon above the bottom polygon (Figure 5D, yellow over pink polygon). Thus, the longitudinal 1D growth of nanowire through the catalyst using VLS process and lateral growth of new polygon along with the increase in size of old polygons with VS process will continue as shown in Figure 5E and 5F. After the critical size, first polygon (yellow f pink f blue polygon) and second polygon (green f yellow f pink polygon) at the bottoms in Figure 5E and 5F do not show further growth. As EDS (Figure 3b and 3c) data shows, the native oxide on the side of the polygons of constant size was more than that of the nanowire wall, it
(oxide) can prevent adatoms to adsorb on the polygons. Thus as the rate of desorption from the polygons start increasing, they enter into the space and might get adsorbed on the above polygons having low oxide concentration. As the size of polygons increase with time the native oxide on the side will also increase and at a specific size the concentration of oxide at the side wall of the polygons is such that it will almost stop the further growth of polygons. At this stage the amount of adatoms will be desorbed due to the mounting oxide thickness on the side wall of the polygons and that is why the stems of the NTrs constitute constant size of polygons. During all this process, the overall length of the nanowire i.e. the critical length (LC) remains constant. The schematic of 3D crystal model of polygon (Figure 5G) grown above the critical length (LC) has been constructed from the observations of FESEM and TEM images of Cr-doped Ge NTrs. The green color filled area represents {100} faces and the red color filled area represents the {111} faces. The schematic has revealed that the growth of the polygons occur on the wall of Ge {111} plane and the polygon surfaces are bounded with alternating {111} and {100} faces. The same kind of faceting was observed on the Si {112} NW sidewalls due to the presence of boron impurities,39 which suggests that Cr doping in Ge nanowires can be responsible for growing facets around the NW wall.
’ MAGNETIC PROPERTIES The static magnetic properties of Cr-doped germanium NTrs have been investigated by means of SQUID magnetometry. Figure 6a shows temperature dependent magnetization (M) of 2961
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NTrs, the germanium oxide layer around stacking polygons has been mostly removed by dipping the as-prepared sample in water for 20 min. The removal of most germanium oxide has been confirmed by XRD analysis (Supporting Information Figure S3). The ferromagnetic properties (TC, HC, MR, and MS) in both the cases (before and after removal of germanium oxide) were observed to be the same, which suggests that Cr-doped Ge NTrs can contribute in room temperature ferromagnetic ordering, which is reasonable since germanium oxide is diamagnetic and hence it can not contribute in FM ordering. Cr may substitute in Ge lattice and so that with interaction with Ge atoms it can generate FM ordering in Ge NTrs. As the presence of secondary phases or inhomogeneities and surface morphology43a,b of nanostructures could be a crucial feature in increasing the values of ferromagnetic characteristics, the Cr substitution in germanium NTrs would be more effective to improve ferromagnetic ordering compared to bulk single crystal of Ge0.99Cr0.01. This significant room temperature ferromagnetism and enhancement in its values may offer very attractive magnetotransport properties which need to be investigate in detail.
Figure 6. (a) MT curve, (b) MH curves, and (inset) low-field magnification of the hysteresis curves, for NTrs sample after removing GeO2.
NTrs between 5 and 300 K in a 100 Oe magnetic field. The diamagnetic component arising from silicon substrate was subtracted from the hysteresis loops. The magnetization for NTrs shows a phase transition at 224 K. Clear hysteresis loops (Figure 6b) were observed for NTrs by measuring field dependent M-H curves at room temperature (300 K), 100 and 5 K. The inset of Figure 6b indicating that, the average coercive field (HC) of as-prepared NTrs samples at 300 K, 100 and 5 K are 35 Oe, 118 and 208 Oe, respectively. The average remanences (MR) at 300 K, 100 and 5 K are 0.00195 emu/g 0.00665 emu/g and 0.014 emu/g, respectively. Also the saturation magnetization (MS) is observed to decrease with increasing temperature from 5 to 300 K. Thus the obtained values of HC, MR, and MS have been increased with decrease in temperature, which illustrate the temperature dependence of the magnetic moment. As previously reported, Cr-doped in bulk single crystalline Ge exhibited a ferromagnetic transition at TC of 126 K, which is attributed to the presence of secondary phases (inhomogeneous Cr inclusions in Ge matrix).37a However, from recent investigation on magnetic properties of Ge0.99Cr0.01 NWs, it was demonstrated that the large proportion of diluted Cr ions was precipitated in antiferromagnetic nanoclusters with mutually compensated magnetic moments.37b In the case of Cr-doped Ge NTrs, the ferromagnetic ordering was observed at room temperature. It is interesting to note that, in the case of NTrs, the obtained values of HC and MR at room temperature were found to be far larger than that of Cr-doped bulk single crystal Ge at 5 K. To clarify the contribution of ferromagnetism in Cr-doped germanium
’ SUMMARY AND CONCLUSIONS In summary, we have successfully synthesized Cr-doped Ge NTrs using a vapor transport method. Experiments for growth of Ge nanostructure were carried out with and without Cr source, which suggest that Cr can play the role to change the morphology of the nanostructures. For better understanding of growth mechanism, the reaction time was varied by keeping other experimental parameters constant. The proposed mechanism was based on VLS and VS mechanisms. The initial 1D growth of a Ge nanowire-doped with Cr atoms was due to VLS process and the growth and stacking of polygonal plates to form nanotower was formed by VLS þ VS mechanism. The morphology of a Crdoped Ge NTr was found to depend on the diffusion length of Ge atoms in the presence of dopant (Cr atoms). The room temperature ferromagnetism in Cr-doped Ge NTrs was observed. The improvement in magnetotransport properties could meet the criteria to be an effective DMSs material. Thus it is plausible that the anisotropic morphological material such as NTrs could act as a very promising material for future spintronics applications. ’ ASSOCIATED CONTENT
bS
Supporting Information. Figures showing detail scans of Cr 2p3/2 and 2p1/2 peaks, TEM image of stem of Ge NTr, and the XRD spectrum of Cr-doped NTrs after removal of germanium oxide. This information is available free of charge via the Internet at http://pubs.acs.org/.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors acknowledge the National Science Council, Taiwan (ROC) for the support through a Grant No NSC 982221-E-007-104-MY3. 2962
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