General Control of Transition-Metal-Doped GaN Nanowire Growth

Aug 7, 2008 - We report the first synthesis and characterization of cobalt- and chromium-doped GaN nanowires (NWs), and compare them to ...
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NANO LETTERS

General Control of Transition-Metal-Doped GaN Nanowire Growth: Toward Understanding the Mechanism of Dopant Incorporation

2008 Vol. 8, No. 9 2674-2681

Kevin G. Stamplecoskie, Ling Ju, Shokouh S. Farvid, and Pavle V. Radovanovic* Department of Chemistry, UniVersity of Waterloo, 200 UniVersity AVenue West, Waterloo, Ontario, N2L 3G1, Canada Received April 3, 2008; Revised Manuscript Received June 17, 2008

ABSTRACT We report the first synthesis and characterization of cobalt- and chromium-doped GaN nanowires (NWs), and compare them to manganesedoped GaN NWs. Samples were synthesized by chemical vapor deposition method, using cobalt(II) chloride and chromium(III) chloride as dopant precursors. For all three impurity dopants hexagonal, triangular, and rectangular NWs were observed. The fraction of NWs having a particular morphology depends on the initial concentration of the dopant precursors. While all three dopant ions have the identical effect on GaN NW growth and faceting, Co and Cr are incorporated at much lower concentrations than Mn. These findings suggest that the doping mechanism involves binding of the transition-metal intermediates to specific NW facets, inhibiting their growth and causing a change in the NW morphology. We discuss the doping concentrations of Mn, Co, and Cr in terms of differences in their crystal-field stabilization energies (∆CFSE) in their gas-phase intermediates and in substitutionally doped GaN NWs. Using iron(III) chloride and cobalt(II) acetate as dopant precursors we show that the doping concentration dependence on ∆CFSE allows for the prediction of achievable doping concentrations for different dopant ions in GaN NWs, and for a rational choice of a suitable dopant-ion precursor. This work further demonstrates a general and rational control of GaN NW growth using transition-metal impurities.

Doping semiconductor nanostructures with selected dopant ions represents an effective means of imparting new electrical,1 optical,2,3 and magnetic4 properties into these promising materials, thereby expanding their intrinsic functionalities. In particular, nanostructured diluted magnetic semiconductors (DMSs),5 obtained by substitutional doping of nanocrystalline compound semiconductors with paramagnetic transitionmetal ions, can enable simultaneous control and manipulation of electron spins and charges at the nanoscale. This approach is pivotal in the formulation of spin-based electonics, or spintronics, which has emerged as a potential alternative to conventional charge-based electronics.6 Semiconductor nanowires (NWs) have been proposed or demonstrated as very promising components for future electronic,7,8 photonic,9,10 and biosensing11 nanodevices. Diluted magnetic semiconductor nanowires (DMS-NWs)2,12-16 have, therefore, attracted significant interest in recent years due to the possibility of expanding devices’ functionality using electronic spin as an additional degree of freedom. Moreover, shape anisotropy of DMS-NWs may potentially allow for a reduced magnetostatic energy and, therefore, easier magnetization of the NWs along the growth direction.17 Among different DMS* Corresponding author. E-mail: [email protected]. 10.1021/nl8009523 CCC: $40.75 Published on Web 08/07/2008

 2008 American Chemical Society

NW systems studied thus far Mn-doped GaN (Mn:GaN) NWs have drawn particular attention due to proposed ferromagnetic ordering of Mn ions in the GaN lattice above room temperature,18 and the excellent optical and electrical properties of GaN NWs.7,9 This material has gradually emerged as a model system for studies of magnetic semiconductor NWs. While several research groups have reported syntheses of Mn:GaN NWs,2,13-15 systematic studies of GaN NW growth in the presence of Mn impurities and of the mechanism of dopant ion incorporation have been very limited.14 We have recently demonstrated that the morphology and faceting of Mn:GaN NWs is dopant ion concentration dependent, and that NW growth direction and faceting can be controlled by controlling the concentration of Mn precursor in the reaction mixture.14 Subsequently, Hwang et al.15 showed that temperature can also be used to control the growth direction of Mn:GaN NWs. Understanding the NW growth in the presence of impurities and the mechanism of dopant ion incorporation is critical for a rational synthesis of DMS-NWs, and for predictions of the systems which can be prepared under given experimental conditions. The main question in this context is what governs the incorporation of dopant ions into NWs having particular morphologies. The answer to this question requires a thorough investigation

of GaN DMS-NW growth and dopant incorporation, which must involve comparative studies of doping GaN NWs with different transition metals. Preparation of various GaN-based DMS-NWs would also allow for studies of their functionality with respect to the nature of the dopant ions, and the NW growth and morphology. Furthermore, one-dimensional growth and well-defined faceting of NWs represents a powerful tool for general studies of crystal growth in the presence of inorganic impurities. Aside from Mn:GaN NWs, however, there have been few experimental reports of other GaN DMS-NW systems.16 In this Letter we report the synthesis and characterization of cobalt- and chromium-doped GaN NWs, and compare the findings to those for Mn-doped GaN NWs. We observed three distinct NW cross-section morphologies: hexagonal, triangular, and rectangular. The fraction of triangular and rectangular NWs in the reaction product is found to increase with precursor concentrations at the expense of hexagonal NWs. While all three dopant ions have the identical effect on GaN NW growth and faceting, Co and, particularly, Cr are incorporated at much lower concentrations than Mn. Our results suggest that the doping mechanism generally involves binding of the dopant ion intermediates to the specific NW facets, which inhibits their growth and causes a change in the NW morphology. The difference in doping concentrations of Mn, Co, and Cr is discussed in terms of their crystalfield stabilization energies (CFSEs) in corresponding intermediates and in substitutionally doped GaN NWs. This work demonstrates general control of GaN NW growth direction using transition-metal impurities, and allows for a rational choice of dopant precursors for a particular GaN DMS-NW system. Transition-metal-doped GaN NWs were synthesized by chemical vapor deposition (CVD) method, as described previously.2,14 The syntheses were performed from elemental gallium and varying amounts of dopant-ion precursors in a flow of gaseous ammonium and hydrogen, in a 2 in. threezone tube furnace. All samples were prepared on sapphire substrates at 990 °C under identical gas flow rates using MnCl2, CoCl2, and CrCl3 as dopant precursors. The substrates were placed about 1 cm downstream of the solid precursors. Nickel nanoparticles, prepared in situ from Ni(NO3)2 deposited on the substrates as ethanol solutions, were used as growth catalysts. The reaction was allowed to proceed for 20 min, at which point the gas flow was stopped, and the reaction products were allowed to slowly cool down under argon flow, after evacuation of the system. The samples were studied with scanning electron microscope (SEM, LEO 1530), high-resolution transmission electron microscope (HRTEM, JEOL 2010F), and X-ray diffraction (XRD, Bruker AXS D8). A representative overview SEM image of Co-doped GaN NWs is shown in Figure 1a. Similar images were obtained for Cr-doped GaN NWs (Figure S1, Supporting Information). Nanowires can be obtained in a very high yield having a typical length on the order of micrometers (often several tens of micrometers). While pure GaN NWs, synthesized in the absence of transition-metal precursors, exhibit exclusively Nano Lett., Vol. 8, No. 9, 2008

Figure 1. (a) Overview SEM image of Co:GaN NWs synthesized with 10 mol % of CoCl2 with respect to Ga in the reaction mixture. (b, c) SEM images of typical triangular and rectangular Co:GaN NWs, respectively. (d) XRD pattern of Co:GaN NWs (lines indicate bulk hexagonal GaN reflections).

hexagonal cross sections, a fraction of the transition-metaldoped NWs always have triangular and rectangular cross sections (Figure S2, Supporting Information). Representative SEM images of triangular and rectangular Co-doped GaN NWs are shown in Figures 1b and 1c, respectively. The crystal structure of these NWs is hexagonal, as evidenced from the XRD pattern shown in Figure 1d. The observed XRD reflections can be assigned to a wurtzite GaN crystal structure. A careful inspection of all XRD data also reveals other small peaks, some corresponding to transition-metal oxides or nitrides codeposited on the growth substrates. This observation suggests that magnetic measurements on the growth substrates, reported13,15,16 for doped GaN NWs prepared using similar methodologies, must be considered with significant caution. Figure 2 shows typical HRTEM images of Co:GaN NWs having triangular (Figure 2a) and rectangular (Figure 2b) cross sections. The corresponding electron diffraction (ED) patterns are shown as insets. The HRTEM images reveal that NWs are single crystalline and have smooth surfaces and uniform thicknesses. No secondary phases related to dopant ions were observed in any of the NWs imaged along their entire lengths. Similar images were obtained for Cr: 2675

Figure 3. EDX Ga (red line) and Co (yellow line) line scan profiles of (a) triangular and (b) rectangular Co:GaN NWs from Figure 2. Co profiles are multiplied by 50 for clarity.

Figure 2. HREM images of (a) triangular and (b) rectangular Co: GaN NWs showing NW growth directions. Insets: corresponding electron diffraction patterns.

GaN NWs (Figure S3, Supporting Information). These observations are consistent with our previous findings for Mn-doped GaN NWs.14 The ED pattern of the triangular NW (Figure 2a, inset) was recorded along the [0001] zone axis, and the rectangular NW was imaged along the [21j1j0] zone axis. These ED patterns allow for the determination of the growth directions as 〈011j0〉 type, similar to Mn:GaN NWs.14 This is in stark contrast to pure GaN NWs synthesized in the absence of transition-metal precursors, which exhibit exclusively hexagonal cross sections and grow along the 〈0001〉 direction.14 The doping concentrations and the dopant ion distributions were measured with energy dispersive X-ray (EDX) spectroscopy. Typical EDX spectra of single Co:GaN and Cr: GaN NWs are shown in Figure S4 (Supporting Information). The average doping concentrations for Mn-, Co-, and Crdoped GaN NWs were estimated to be ca. 1.4%, 0.5%, and 0.2%, respectively.19 These concentrations are well below the reported solubility limits of the corresponding dopants in the GaN lattice.20 The doping concentrations of each transition-metal are found to be comparable for all three NW morphologies (hexagonal, triangular and rectangular), and for different starting concentrations of dopant precursors in the reaction mixtures. Although there is variation in doping 2676

concentrations for individual NWs, there is a clear difference in the average doping concentrations of Mn, Co and Cr dopants. EDX elemental line scan profiles of triangular and rectangular Co:GaN NWs from Figure 2, recorded perpendicularly to the NW growth directions, are shown in Figures 3a and 3b, respectively. The difference between EDX line scan profiles of surface-bound and homogeneously distributed dopants in NWs has been documented, and EDX elemental line-scan profiling can serve as a valuable tool for determining dopant ion locale in a single NW.2 The Co profiles are essentially identical to the Ga profiles (also Figure S5, Supporting Information), indicating a homogeneous distribution of dopant ions across these NWs, similarly to Mn:GaN NWs described previously.14 Although Cr doping concentrations are generally very small, often below detection limit, for NWs in which the doping concentrations are sufficiently high, Cr EDX line scan profiles also appear to be identical to those of Ga (Figure S6, Supporting Information). While EDX line scan profiles suggest that most of the loosely NW surface-bound dopant ions are desorbed in the cooling stage of sample preparation, it cannot be excluded that for very low doping concentrations the majority of detected dopants are bound to the NW surfaces. Figure 4 shows a percentage of NWs having a specific cross section as a function of dopant ion concentration in the reaction mixture. For all three transition-metal dopants the number of hexagonal NWs decreases with increasing initial dopant precursor concentration (red circles), becoming negligible for dopant concentrations in the reaction mixture above 25 mol %. The fraction of NWs with triangular cross sections initially increases reaching the maximum values for dopant precursor concentrations between 10 and 15 mol %, followed by a steady decline (blue triangles). The fraction of rectangular NWs is, on the other hand, negligible for pure GaN NWs, but steadily increases with dopant precursor Nano Lett., Vol. 8, No. 9, 2008

Figure 4. Percent of hexagonal (red circles), triangular (blue triangles), and rectangular (green squares) NWs vs dopant ion precursor concentration for (a) Mn:GaN, (b) Co:GaN, and (c) Cr: GaN NWs.

concentration, becoming dominant at high concentrations (∼25 mol %). Consecutive increase in dopant precursor concentration resulted in a very low NW yield, preventing the studies at higher precursor concentrations. Clearly, the number of hexagonal NWs decreases concurrently with an increase in the number of triangular and, eventually, rectangular NWs. We have observed identical effects using other precursors of the same dopant ions. Specifically, with cobalt(II) acetate (Co(OAc)2) as the Co precursor, identical trends in NW morphology with respect to dopant precursor concentrations were observed, indicating that the coordinating anions do not have a significant effect on the NW growth and faceting. This observation suggests that the doping mechanism generally involves a critical step of dopant intermediate adsorption to the specific surfaces of GaN NWs. Dopant incorporation, therefore, occurs differently than the metal nanoclyster catalyzed NW growth, which is most often suggested to be based on a vapor-liquid-solid (VLS) mechanism. Furthermore, this change in NW faceting with respect to dopant precursor concentration demonstrates a general control of GaN NW growth using transition-metal dopants. We also observed an increase in the number of triangular and rectangular GaN NWs synthesized in the presence of MgCl2, further confirming the general control of GaN NW growth using small inorganic impurities. As magnesium is not a transition-metal dopant, and is traditionally used to impact the electrical rather than magnetic properties of GaN (leads to p-type GaN),21 this observation is beyond the consideration of the current work. Based on the HRTEM and ED data we can begin to identify the facets to which dopant ions bind during incorporation, in order to eventually elucidate the doping mechanism (Figure 5a). Hexagonal NWs, which are the most Nano Lett., Vol. 8, No. 9, 2008

Figure 5. Schematics of NW faceting and dopant incorporation. (a) Faceting of hexagonal, triangular and rectangular GaN NWs. (b) Influence of dopant precursor concentration on NW faceting shown for Mn:GaN NWs. (c) Difference in incorporation of Mn, Co, and Cr dopants shown for rectangular NWs.

stable in the absence of impurities, have six equivalent facets of the type {101j0}. In the presence of transition-metal precursors the dopant intermediates bind to these exposed facets, or their edges, and inhibit their growth. At low dopant precursor concentrations new {21j1j2j} and {0001} type facets appear as a consequence giving rise to the change in the NW cross section from hexagonal to triangular. The subsequent growth of these new facets leads to the formation of the fully triangular NWs. At high precursor concentrations the adsorbed or bound dopant intermediates inhibit the growth of other facets, allowing for the growth of {21j1j0} and {0001} facets, and the formation of the rectangular NWs. Further increase in dopant precursor concentrations leads to the complete termination of the NW growth. The effect of transition-metal dopants on the faceting of GaN NWs is shown schematically in Figure 5b, using Mn: GaN NW as an example. With increasing amount of dopant precursors in the reaction mixtures, the dopant ion gas-phase intermediates can bind to the facets or edges formed during NW nucleation and inhibit their growth. At some critical number of surface-bound dopant intermediates a complete termination of the growth of corresponding facets occurs, causing the appearance of other facets, and a change in the NW growth direction and morphology. Dopant intermediates in the gas phase can then bind to the sites on newly formed facets. Growth of such facets, as the NWs grow in width, allows for incorporation of the surface-bound dopant ions. A similar modification of the crystal faceting and shape by small inorganic molecules has been observed in biominerals.22 It is very interesting, however, that an increase in the 2677

dopant precursor concentration has a strong effect on the NW faceting, but not on the doping concentrations of a particular dopant in NWs having different morphologies. It is conceivable that the effect of higher dopant precursor concentrations is compensated by the lower adsorption energies of dopant intermediates on the newly formed NW facets or edges. Our work also shows that wurtzite III-V nanostructures can indeed be doped from the gas phase through dopant binding to {101j0}, {112j0}, and {0001} facets. These facets have been suggested to provide unfavorable Mn binding sites on II-VI wurtzite lattices, which has been associated with the failure to dope CdSe nanocrystals from solution.23 Given the same effect of Mn, Co, and Cr dopants on the faceting of GaN NWs, it is likely that all dopant intermediates adsorbed on the particular NW surfaces have comparable residence times. While the Mn, Co, and Cr precursors have the same effect on GaN NW faceting, the average doping concentrations of these metal ions are different. The dopant ion incorporation is clearly related to the nature of the dopant ions and follows the pattern Mn > Co > Cr (Figure 5c). The doping concentration limits must be determined by the energy required for transformation from surface-bound dopant intermediate species to internally incorporated dopant ions. In the case of transition-metal ions this energy can be considered in terms of crystal-field stabilization energies (CFSEs) of dopant ions in surface-bound intermediates and in the GaN NW lattice. The dopant ions examined in this work exhibit an octahedral geometry in their chloride precursors (MnCl2, CoCl2, and CrCl3). The substitutional incorporation of dopant ions into the GaN NW lattice requires that the dopant ions change their coordination numbers from six to four, and assume a tetrahedral geometry. This geometrical change is accompanied by a change in the electronic configuration of the transition-metal d orbitals (Figure 6a). Specifically, the octahedral crystal-field-split d orbitals (eg and t2g) are separated by the quantity 10Dq, with t2g levels lowered by 4Dq, and eg levels raised by 6Dq from the barycenter. In the tetrahedral crystal-field this splitting is inverted leading to the two sets of d orbitals, e and t2, with opposite energy level schemes. The distribution of electrons in these energy levels for a particular geometry, transition-metal ion, and coordinating ligands determines the CFSE. In a weak octahedral field each electron entering the t2g levels stabilizes the transition-metal ion by -4Dq, while each electron entering the eg levels destabilizes the ion by 6Dq. Similarly, for tetrahedral complexes electrons entering the e levels stabilize the coordinated metal ions by -6Dq, while the electrons entering the t2 levels destabilize them by 4Dq. The difference between the CFSE of the dopant ions in tetrahedral geometry in the GaN lattice and the CFSE of those ions bound to the NW facets is related to the doping concentration limits in NWs. Assuming that the dopant ions in the gas phase are bound to the NW facets in the modified precursor form, the CFSE can be estimated for Mn, Co, and Cr ions in surface-bound (octahedral) and internally doped (tetrahedral) coordination, neglecting the configuration interactions. We plotted doping concentration dependence on 2678

Figure 6. (a) Schematics of transformation of transition-metal (TM) geometry from octahedral (in GaN NW-bound intermediates) to tetrahedral (substutionally doped in GaN NW), and of the corresponding d orbital splittings. TM intermediates coordinated with Cl are shown as an example. (b) Average doping concentration dependence on the difference in crystal-field stabilization energies (∆CFSE) of dopants in intermediates, originated from precursors indicated in the graph, and in GaN NWs. Dashed line is an empirical fit to the points obtained for MnCl2, CoCl2, and CrCl3 precursors (red dots). This dependence is tested by doping GaN NWs using different transition-metal precursors (green triangles). ∆CFSE is calculated for Mn3+, Co2+, Cr3+, and Fe3+ oxidation states in GaN NWs.

the CFSE difference (∆CFSE ) |CFSE(Oh)| - |CFSE(Td)|) for the Mn, Co, and Cr dopants in Figure 6b. The CFSE values of octahedral transition-metal ions in MnCl2 (d5 system), CoCl2 (d7 system), and CrCl3 (d3 system) used as dopant precursors are 0Dq, -8Dq, and -12Dq, respectively. The corresponding Dq values of metal ions in these compounds are 830, 690, 1370 cm-1 for MnCl2,24 CoCl2,25 and CrCl3,26 respectively. The dopant intermediates in the gas phase might in this case also be partly or fully coordinated with NH3. The Dq values used for calculating the CFSE in Figure 6b are obtained as averages between Dq values for dopant ions coordinated with six Cl and six NH3 ligands. To our knowledge, the exact coordination of dopant intermediates in the gas phase under the conditions relevant for this analysis is not known. All three transitionmetal ions, particularly Cr3+, however, readily adopt octahedral geometry.27 Importantly, even if Mn2+ and Co2+ were tetrahedral in the gas phase, the corresponding data points in Figure 6b would be within the error bars of the existing points.28 The ∆CFSE independence on the dopant intermediate coordination for the particular dopant ions studied in this Nano Lett., Vol. 8, No. 9, 2008

work strongly supports our model (Vide infra). Estimations of the CFSE for tetrahedral Mn, Co, and Cr ions substitutionally doped into GaN lattice are somewhat less certain due to the lack of experimental data of these parameters and of the exact oxidation states of particular dopant ions in the GaN lattice. It has been suggested that Mn dopants have Mn3+ character in GaN.2,29 For Co:GaN NWs we assume the Co2+ oxidation state in GaN NWs, as tetrahedral Co3+ is an extremely unusual species.27 Even in the unlikely case of cobalt being doped in GaN NWs as Co3+ (d6), the ∆CFSE would be within the error bar indicated for CoCl2 in Figure 6b (Vide infra). Similarly, Cr3+ is very stable, does not readily undergo redox reactions,27 and is therefore likely to remain in the same oxidation state in GaN.30 Based on these considerations we estimated the CFSE values for these dopant ions in GaN NW lattice to be -4Dq for Mn3+(d4), -12Dq for Co2+(d7), and -8Dq for Cr3+(d3). The Dq values for these ions in tetrahedral environment were estimated to be 850, 450, and 800 cm-1 for Mn3+, Co2+, and Cr3+ dopants respectively,27,31 based on the literature values for coordination complexes with similar local environments to GaN.32 The error bars in Figure 6b reflect a reasonable uncertainty in doping concentrations, coordination of dopant ion intermediates, and tetrahedral crystal-field strength of the GaN lattice.33 With a better knowledge of these parameters the data points in Figure 6b could be more accurately determined, which would improve this model. The average doping concentration dependence on ∆CFSE can be empirically fit well to an exponential function (dotted line).34 This dependence can be used to predict the doping concentrations of different transition metals in GaN NWs, and to rationally select the best precursors for doping NWs with particular dopant ions. We have tested this hypothesis by doping GaN NWs using different transition-metal precursors. Synthesis of Fe:GaN NWs using FeCl3 yielded the average doping concentration of 0.6%, very similar to Co dopants. For Fe3+ (d5 system) ∆CFSE is zero, and the average doping concentration is in very good agreement with the predicted concentration based on the established dependence (green triangle labeled FeCl3 in Figure 6b). Furthermore, doping GaN NWs with Co using Co(OAc)2 yielded the average doping concentration of 0.4%, somewhat lower than when CoCl2 was used. The Dq value of surface-bound dopant intermediates used for calculating the ∆CFSE is estimated as an average between the Dq of Co2+ ion coordinated with six acetate and six NH3 ligands. The corresponding data point, shown as a green triangle labeled Co(OAc)2 in Figure 6b, falls precisely on the fit curve. Importantly, the empirical function obtained by fitting all data, including these two additional test data points, is in very good agreement with the dependence obtained with the three original data points (Figure S7, Supporting Information),34 further strengthening our hypothesis. Additional corroboration includes the fact that Cr3+ complexes are nearly completely restricted to six-coordinate complexes,27 which explains the low doping concentrations of Cr3+ ions in GaN NWs. For comparison we also considered Mn dopants in the 2+ oxidation state (d5 system), in which case ∆CFSE ) Nano Lett., Vol. 8, No. 9, 2008

0, identical to that of Fe3+. Based on the empirical dependence reported here the doping concentration of Mn2+ should be much lower than that of Mn3+, consistent with the previous suggestion2 that Mn dopants in GaN NWs are largely in the 3+ oxidation state. It is reasonable to suggest that other thermodynamic factors may also be responsible for dopant incorporation into GaN NWs, most notably a solubility of dopant ions in the nickel nanoclyster catalyst, concentrations of different dopant ions in the gas phase under the given synthetic conditions, and the difference in ionic radii of dopant ions and the Ga3+ host cation. An inspection of phase diagrams35 reveals that all dopant ions have full solid solubility in nickel at the concentrations of dopant precursors used in this work, indicating that dopant solubility is not the determining factor for the difference in doping concentrations. Furthermore, iron nanoclusters have also been used as catalysts for GaN NW growth, but no iron was detected in the GaN NWs even in such an extreme case.36 Although the vapor pressures of the dopant precursors are, to our knowledge, not reported, the boiling points of MnCl2, CoCl2, and CrCl3 are 1190 °C, 1050 °C, and 1300 °C, respectively.37 This temperature range is too narrow to provide significantly different concentrations of dopant precursors in the gas phase, and could not account for the observed difference in doping concentrations. In spite of the much lower boiling point of FeCl3 (∼315 °C),37 the average doping concentration of Fe is only ca. 0.6%, suggesting that dopant incorporation is not limited by the concentration of precursors in the gas phase. Similarly, the melting temperature of Co(OAc)2, which is the only relevant constant we could find for this compound, is ca. 300 °C compared to 740 °C for CoCl2,37 even though the average Co doping concentrations are very similar when these two precursors are used. Finally, there is no systematic dependence of ionic radii of dopant ions on the observed doping concentrations. This can be illustrated by considering the ionic radii of tetrahedral Co2+ (∼0.58 Å) and Fe3+ (∼0.49 Å), which have similar average doping concentrations, with respect to the radius of Ga3+ host cation (∼0.47 Å).38 While ionic radius data for tetrahedral Mn3+ and Cr3+ are not reported, their ionic radii in octahedral geometry (which usually scale to those of tetrahedral geometry) are very similar (∼0.64 Å for Mn3+ vs 0.61 Å for Cr3+),38 despite their large difference in doping concentrations. From the above considerations, we believe that both the change in the NW morphology and the doping concentration limits originate from binding of the dopant intermediates from the gas phase to the NW surfaces, but the two effects do not seem to have a direct correlation. Adsorption and binding of the dopant impurities from the gas phase to the NW surfaces, which results in a systematic change in NW morphology, is a general phenomenon, but the degree of dopant incorporation depends on the nature of the dopant ion and its coordination in the intermediate form. While this work deals with transition-metal dopants for which energy differences between surface-bound and internally incorporated dopant ions can be described by the ligand-field theory, we believe that the dopant incorporation based on energy differences 2679

between surface-bound and internally incorporated dopants is a general doping mechanism of NWs prepared by the CVD method. The general approach to NW doping, in the context of our results, is to destabilize NW-bound dopant intermediates with respect to their incorporation into NW lattice. Other doping mechanisms may also be possible for particular dopant-NW systems, including incorporation of dopants through VLS mechanism for the systems with the appropriate phase diagrams. In summary, Co- and Cr-doped GaN NWs have been prepared, and compared to Mn-doped GaN NWs. The effect of these transition-metal dopants on the growth and faceting of GaN DMS-NWs has been described. MnCl2, CoCl2, and CrCl3 precursors alter the GaN NW growth and faceting in the same way; as dopant precursor concentrations in the reaction mixtures increase, so do the fractions of triangular and rectangular NWs at the expense of hexagonal NWs. This observation suggests that dopant ion incorporation generally occurs by binding of the dopant intermediates to the exposed NW facets, which inhibits their further growth, and that residence times of surface-bound intermediate species are similar for all three dopant ions. The structural analysis allowed for identification of the faceting of hexagonal, triangular, and rectangular NWs. Internal doping is achieved for all three dopant ions studied, but the average doping concentrations follow the order Mn > Co > Cr. The difference in doping concentrations has been related to the transformation of dopant ions from surface-bound intermediates to internally incorporated species. This transformation is associated with the difference between the crystal-field stabilization energies of the dopant ions in NW-bound dopant intermediates and the substitutional dopants in GaN NW lattice. The doping concentration dependence on ∆CFSE allows for prediction of possible doping concentrations of particular transition-metal ions, and for the rational choice of the suitable dopant precursors. Furthermore, the morphology of the DMS-NWs can be controlled using appropriate starting amounts of dopant precursors. The generality of controlling growth and faceting in the presence of dopant precursors clearly shows that judicious choice of impurities can also be used to control the growth direction and faceting of pure GaN NWs without any impurity incorporation. This methodology enables the preparation and study of both pure and doped GaN NWs with respect to structure, composition and morphology. In a broader context, these results can be applied for the design and preparation of new multifunctional 1-D nanostructures for spintronics and other electronic technology applications. Acknowledgment. We thank NSERC (Discovery and RTI programs) and the University of Waterloo (startup funding) for the financial support of this work. P.V.R. is Canada Research Chair in physical chemistry and spectroscopy of nanoscale materials. We thank Fred Pearson (Brockhouse Institute for Materials Research at McMaster University) for technical assistance with TEM imaging and EDX measurements. Supporting Information Available: SEM and HRTEM images of Cr:GaN NWs; EDX spectra of single Co:GaN and 2680

Cr:GaN NWs; normalized EDX line scan profiles of Co: GaN NWs; EDX line scan profiles of Cr:GaN NW; doping concentration vs ∆CFSE. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Zhong, Z.; Qian, F.; Wang, D.; Lieber, C. M. Nano Lett. 2003, 3, 343. Chen, X.; Lee, S. J.; Moskovits, M. Appl. Phys. Lett. 2007, 91, 082109. (2) Radovanovic, P. V.; Barrelet, C. J.; Gradecak, S.; Qian, F.; Lieber, C. M. Nano Lett. 2005, 5, 1407. (3) Bol, A. A.; Meijerink, A. Phys. ReV. B 1998, 58, 15997. Boyer, J.-C.; Vetrone, F.; Cuccia, L. A.; Capobianco, J. A. J. Am. Chem. Soc. 2006, 128, 7444. Ohno, Y.; Young, D. K.; Beschoten, B.; Matsukura, F.; Ohno, H.; Awschalom, D. D. Nature 1999, 402, 790. (4) Radovanovic, P. V.; Gamelin, D. R. Phys. ReV. Lett. 2003, 91, 157202. Matsumoto, Y.; Murakami, M.; Shono, T.; Hasegawa, T.; Fukumura, T.; Kawasaki, M.; Ahmet, P.; Chikyow, T.; Koshihara, S.; Koinuma, H. Science 2001, 291, 854. Archer, P. I.; Santangelo, S. A.; Gamelin, D. R. Nano Lett. 2007, 7, 1037. (5) Furdyna, J. K. J. Appl. Phys. 1988, 64, R29. (6) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.; von Molnar, S.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Science 2001, 294, 1488. (7) Li, Y.; Qian, F.; Xiang, J.; Lieber, C. M. Mater. Today 2006, 9, 18. Li, Y.; Xiang, J.; Qian, F.; Gradecak, S.; Wu, Y.; Yan, H.; Blom, D. A.; Lieber, C. M. Nano Lett. 2006, 6, 1468. (8) Tian, B.; Zheng, X.; Kempa, T. J.; Fang, Y.; Yu, N.; Yu, G.; Huang, J.; Lieber, C. M. Nature 2007, 449, 885. (9) Qian, F.; Li, Y.; Gradecak, S.; Wang, D.; Barrelet, C. J.; Lieber, C. M. Nano Lett. 2004, 4, 1975. Pauzauskie, P. J.; Sirbuly, D. J.; Yang, P. Phys. ReV. Lett. 2006, 96, 143903. (10) Duan, X.; Huang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421, 241. Law, M.; Sirbuly, D. J.; Johnson, J. C.; Goldberger, J.; Saykally, R. J.; Yang, P. Science 2004, 305, 1269. (11) Patolsky, F.; Zheng, G.; Lieber, C. M. Anal. Chem. 2006, 78, 4260. Patolsky, F.; Timko, B. P.; Yu, G.; Fang, Y.; Greytak, A. B.; Zheng, G.; Lieber, C. M. Science 2006, 313, 1100. (12) Kar, S.; Santra, S.; Heinrich, H. J. Phys. Chem. C 2008, 112, 4036. Yuhas, B. D.; Fakra, S.; Marcus, M. A.; Yang, P. Nano Lett. 2007, 7, 905. Ronning, C.; Gao, P. X.; Ding, Y.; Wang, Z. L. Appl. Phys. Lett. 2004, 84, 783. Lee, J. Y.; Kim, D. S.; Kang, J. H.; Yoon, S. W.; Lee, H.; Park, J. J. Phys. Chem. B. 2006, 110, 25869. (13) Han, D. S.; Park, J.; Rhie, K. W.; Kim, S.; Chang, J. Appl. Phys. Lett. 2005, 86, 032506. Choi, H.-J.; Seong, H.-K.; Chang, J.; Lee, K.-I.; Park, Y.-J.; Kim, J.-J.; Lee, S.-K.; He, R.; Kuykendall, T.; Yang, P. AdV. Mater. 2005, 17, 1351. Baik, J. M.; Lee, J.-L. J. Vac. Sci. Technol. B 2005, 23, 530. Ham, M.-H.; Myoung, J.-M. Appl. Phys. Lett. 2006, 89, 173117. Deepak, F. L.; Vanitha, P. V.; Govindaraj, A.; Rao, C. N. R. Chem. Phys. Lett. 2003, 374, 314. (14) Radovanovic, P. V.; Stamplecoskie, K. G.; Pautler, B. G. J. Am. Chem. Soc. 2007, 129, 10980. (15) Hwang, S. O.; Kim, H. S.; Park, S.-H.; Park, J.; Bae, S. Y.; Kim, B.; Park, J. Y.; Lee, G. J. Phys. Chem. C 2008, 112, 2934. (16) Seong, H.-K.; Kim, J.-Y.; Kim, J.-J.; Lee, S.-C.; Kim, S.-R.; Kim, U.; Park, T.-E.; Choi, H.-J. Nano Lett. 2007, 7, 3366. (17) McCurrie, R. A. Ferromagnetic Materials Structure and Properties; Academic Press: London, 1994. (18) Dietl, T.; Ohno, H.; Matsukura, F.; Cibert, J.; Ferrand, D. Science 2000, 287, 1019. (19) The average doping concentrations were determined based on EDX measurements on at least ten different NWs for each individual sample synthesized (a total of minimum 30 NWs for each dopant ion). The EDX spectra were collected for 300 s to obtain good signal-to-noise ratio. All spectra were collected in the identical way and with the same instrumental parameters. Although EDX may not be as accurate in determining elemental compositions as some other analytical techniques, it is the only method available to us that allows an estimation of doping concentrations at the single NW level (see also ref 33). Due to the deposition of dopant-related phases along with the NWs on the growth substrates, determination of the doping concentrations by more accurate bulk measurements would not give scientifically valid results. (20) Sedmidubsky, D.; Leitner, J.; Sofer, Z. J. Alloys Compd. 2008, 452, 105. Hashimoto, M.; Emura, S.; Tanaka, H.; Honma, T.; Umesaki, N.; Hasegawa, S.; Asahi, H. J. Appl. Phys. 2006, 100, 103907. Nano Lett., Vol. 8, No. 9, 2008

(21) Kaufmann, U.; Schlotter, P.; Obloh, H.; Kohler, K.; Maier, M. Phys. ReV. B 2000, 62, 10867. (22) De Yoreo, J. J.; Dove, P. M. Science 2004, 306, 1301. Davis, K. J.; Dove, P. M.; De Yoreo, J. J. Science 2000, 290, 1134. (23) Erwin, S. C.; Zu, L.; Haftel, M. I.; Efros, A. L.; Kennedy, T. A.; Norris, D. J. Nature 2005, 436, 91. (24) Pappalardo, R. J. Chem. Phys. 1960, 33, 613. (25) Ferguson, J.; Wood, D. L.; Knox, K. J. Chem. Phys. 1963, 39, 881. (26) Wood, D. L.; Ferguson, J.; Knox, K.; Dillon, J. F., Jr. J. Chem. Phys. 1963, 39, 890. (27) Lever, A. B. P. Inorganic Electronic Spectroscopy; 2nd ed.; Elsevier Science Publishers: Amsterdam, 1984. (28) In tetrahedral Mn2+ all d orbitals are half-filled, and similar to highspin octahedral Mn2+, it also has zero CFSE, giving the identical data point in Figure 6b. Based on our considerations of the possible intermediate coordination in the gas phase, the estimated Dq value of NW-bound tetrahedral Co2+ intermediate would be ca. 450 cm-1, giving a ∆CFSE of zero, well within the error bar of the plotted CoCl2 data point in Figure 6b. (29) Gelhausen, O.; Malguth, E.; Phillips, M. R.; Goldys, E. M.; Strassburg, M.; Hoffmann, A.; Graf, T.; Gjukic, M.; Stutzmann, M. Appl. Phys. Lett. 2004, 84, 4514. (30) Subashchandran, S.; Kimura, S.; Kim, M. S.; Kobayashi, S.; Zhou, Y. K.; Hasegawa, S.; Asahi, H. Jpn. J. Appl. Phys. 2006, 45, 3522. (31) Murakami, Y.; Sakata, K.; Harada, K.; Matsuda, Y. Bull. Chem. Soc. Jpn. 1974, 47, 3021. Cotton, F. A.; Goodgame, D. M. L.; Goodgame, M. J. Am. Chem. Soc. 1961, 83, 4690. Pruchnik, F.; Wajda, S. Rocz. Chem. 1970, 44, 933.

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(32) Dq for tetrahedral Co2+ was estimated based on the Dq value for [Co(NCS)4]2-. Dq for tetrahedral Cr3+ was estimated based on the (4/9)Dq(Oh) value of [Cr(NCSe)6]3-, since tetrahedral Cr3+ compounds are not readily obtained. Dq value for tetradedral Mn3+ was estimated based on (4/9)Dq(Oh) of [Mn(C2O4)3]3-, as the oxalate ligand-field strength is closest in energy to that of NCS- in the spectrochemical series, that we were able to identify. (33) The error bars in Figure 6b were estimated based on the standard deviations of individual NW doping concentrations (y-axis), and of known Dq values (see ref 27) for similar coordination compounds of respective transition-metal ions (x-axis). The uncertainties in doping concentrations estimated in this way are expected to be larger than the uncertainty due to instrumental accuracy. (34) The original points in Figure 6b were best fit to the exponential function y ) 0.462 · exp(-2.827 × 10-4x) + 0.193.The function obtained when using all five experimental data points in the graph is y ) 0.423 · exp(-3.064 ×10-4x) + 0.199. This empirical fit is meant to provide an estimate of doping concentrations expected for different transitionmetal ions in GaN NWs, prepared by the CVD method under similar reaction conditions, using particular dopant precursors. (35) Binary Alloy Phase Diagrams; Massalski, T. B., Ed.; American Society for Metals: Metals Park, OH, 1986; Vols. 1 and 2. (36) Duan, X.; Lieber, C. M. J. Am. Chem. Soc. 2000, 122, 188. (37) Physical Constants of Inorganic Compounds. In CRC Handbook of Chemistry and Physics, Internet Version 2005; Lide, D. R., Ed.; http:// www.hbcpnetbase.com; CRC Press: Boca Raton, FL, 2005. (38) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.

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