Shell Approach to Dopant Incorporation and Shape Control in

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A Core/Shell Approach to Dopant Incorporation and Shape Control in Colloidal Zinc Oxide Nanorods Saahil Mehra, Amy Bergerud, Delia J Milliron, Emory Chan, and Alberto Salleo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00981 • Publication Date (Web): 03 May 2016 Downloaded from http://pubs.acs.org on May 7, 2016

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Chemistry of Materials

A CORE/SHELL APPROACH TO DOPANT INCORPORATION AND SHAPE CONTROL IN COLLOIDAL ZINC OXIDE NANORODS Saahil Mehra1, Amy Bergerud2, Delia J. Milliron3,4, Emory Chan3, Alberto Salleo1,* 1. Department of Materials Science & Engineering, Stanford University, Stanford California 94305, United States 2. Department of Materials Science & Engineering, University of California Berkeley, Berkeley California 94720, United States 3. The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States 4. McKetta Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712, United States  *

Corresponding Author: [email protected]

ABSTRACT Tunable aliovalent doping is critical to controlling the optoelectronic properties of semiconductor nanocrystal systems. However, unintentional dopant-induced shape evolution and kinetically-limited doping reactions in low-temperature nanocrystal syntheses make it difficult to independently control shape and incorporate dopants in colloidal metal oxide nanocrystals. Here, we demonstrate a synthetic strategy for achieving simultaneous control of both nanorod shape and dopant concentration in colloidal zinc oxide nanorods. We show that this approach succeeds in doping zinc oxide nanorods using Group III dopants (indium or aluminum) in varying concentrations, and we quantify the effects of dopant incorporation on the structural, optical, and plasmonic properties of the nanorods. The synthesis of undoped zinc oxide nanorod templates and subsequent addition of dopant salts to the ongoing reaction enables both shape retention and dopant incorporation. Subsequent growth of an undoped shell on the nanorods incorporates surface segregated dopants with high efficiency. This ‘core/shell’ doping strategy presents a general route to achieving controlled dopant incorporation and morphological retention in anisotropic metal oxide nanocrystal systems.

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INTRODUCTION The achievement of compositional control through doping in colloidal semiconductor nanocrystals offers a new dimension to alter nanocrystal physical properties in addition to control of shape and size. Doping, or intentional impurity incorporation, is a common technique used in the bulk to alter the host materials’ structural, magnetic, or optoelectronic properties. In the case of colloidal nanocrystals, recent examples of successful doping strategies include lanthanide doping of rare-earth fluorides for light upconversion1,2 and aliovalently doped metal oxide nanocrystals with tunable absorption properties in the infrared (IR) and near-IR.3-6 Zinc oxide (ZnO) nanocrystals doped with Group III elements (aluminum, gallium, or indium) are of particular interest for optoelectronic applications owing to the low cost and earthabundance of Zn as well as the widespread usage of doped ZnO as a transparent conducting oxide7. The development of strategies to control shape and dopant location in colloidal ZnO nanocrystals would enable fundamental studies of the plasmon shapedependence in this material and would contribute to the rapidly developing field of plasmonic nanocrystals and applications.8-10 Doping colloidal nanocrystals presents its own set of challenges - it often results in unintended nanocrystal shape disruption due to lattice strain when dopants are incorporated within the crystal as well as altered surface chemistry when dopants are confined to the surface8. As a result, highly doped (>5%) nanocrystals often exhibit broader size distributions, particle aggregation, and a loss of shape control relative to their undoped counterparts.10-12 Dispersity and shape control, however, need to be preserved to enable homogeneous physical properties for applications where the spectral

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responses need to have precisely controlled linewidths (for example, imaging, electrochromics, or optoelectronics).8,11,13 For anisotropic zinc oxide nanocrystals, this observed tradeoff between morphology control and dopant incorporation results in nanocrystal dopant concentrations much lower than the bulk solubility limit and restricts the functionality that can be achieved through doping. Thus, the ability to control both of these properties in semiconductor oxide nanocrystals could further enable applications where the nanocrystal plasmon resonances could be tuned by dopant-generated free carriers. Anisotropic nanocrystal shapes present an added challenge for incorporating dopants. Since both shape control and dopant incorporation in colloidal nanocrystals are kinetically controlled phenomena, optimal control of one property can negatively affect control of the other. Nanocrystal shapes are kinetically controlled by manipulating reaction conditions to accommodate the high surface energy to volume ratios in anisotropic nanocrystals. Dopant incorporation is kinetically controlled by precursor reaction rates, since the limited cation diffusion in oxide materials at typical nanocrystal reaction temperatures ( 3:1 CA:Zn precursor). This approach effectively passivated any unfavorable surface configurations upon doping and tuned the relative reactivity of the dopant precursors to facilitate incorporation in as close to equilibrium conditions as possible.10 However, these surfactants are not compatible with growing higher aspect ratio zinc oxide nanorods, so phosphonic acid chemistries were used in this study.16,25 In another approach, undoped nanorods were first grown and an in-situ dopant diffusion step was attempted after the growth reaction (Figure 1B) - analogous to ‘drivein’ steps for dopant incorporation in the semiconductor industry. In this case, we show

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that due to the limited cation diffusion in oxides at low temperatures, the dopants are poorly activated and nearly entirely surface segregated. The limited diffusion at typical reaction temperatures is a doping challenge unique to metal oxide nanocrystals. Due to ionic nature of the zinc oxide crystal structure and fluctuating potential environments created by the host lattice of Zn2+/O2- cations, cation diffusion in zinc oxide at typical colloidal synthesis reaction temperatures (200300°C) is extremely limited in comparison to other metal chalcogenides. For example, the Al3+ diffusion coefficient in ZnO at 900°C is 10-14 cm2/s, which is much lower than the Al3+ diffusion coefficient in the less ionic ZnS (10-7 cm2/s at 900°C).76 Thus, the characteristic diffusion length of Al3+ in ZnO, 4Dt , would be significantly less than 1 nm for a typical four-hour synthesis at 250 °C and an assumed 2.5 eV activation energy for a diffusive hop (scaled using an Arrhenius-like relation).26 As a result, equilibrium distributions of dopant solutes are not achieved since rate processes dictate dopant incorporation and dopants are ‘trapped’ in the crystal at low temperatures. This situation with oxide nanocrystals contrasts greatly with the chalcogenide colloidal nanocrystal systems, where for instance fast diffusion at ambient temperatures enables facile chemical transformation of nanocrystals by cation exchange.27-29 To overcome this doping challenge, we developed a robust synthetic methodology using a ‘core/shell’ doping approach that we focus on in the remainder of this manuscript.

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Figure 1: Schematic of various strategies for doping anisotropic nanorods. Strategies are described as: doping after hot injection (A), in-situ post-growth doping (B), doping after seeds form (C), and ‘core/shell’ type doping (D). Green and black bars denote time over which zinc and dopant precursor solutions, respectively, are injected into the reaction.

A combination of these two approaches yields morphologically controlled, highly doped nanocrystals. The growth of undoped nanorod seeds first enables shape-controlled

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nanorod ‘templates’ to be used as a basis for dopant incorporation (Figure 1C). Dopants were then introduced and subsequently incorporated with high efficiency through the growth of an undoped shell around the nanorod, integrating any surface-segregated dopant cations into the ZnO host lattice (Figure 1D). For metal oxide nanocrystals, this ‘core/shell’ doping approach has been successfully demonstrated for cobalt doped, isotropic ZnO nanoparticles.17 However, it is important to note that the core-shell approach has never been extended to aliovalent dopants or anisotropic nanorod shapes.

Morphological Characterization of Doped ZnO Nanorods The nanorods shown in Figure 2 exhibit stark differences in morphology in nanorods synthesized using the continuous doping (Figure 1A) and core/shell doping (Figure 1D) approaches. ZnO nanorods synthesized with indium present after the initial hot injection resulted in highly branched anisotropic particles and significant disruptions of the nanorod shape. This shape inhomogeneity could be attributed to the strain induced in the lattice by the larger In3+ cation (rion = 94 pm30) relative to Zn2+ (rion = 86 pm30) or the creation of highly reactive, unstable bonding environments when an In3+ cation attaches to a surface site. The low amounts of thermal energy at typical reaction temperatures of 200-300°C could prevent dopant cations from finding an equilibrium bonding environment upon dopant incorporation. Increased branching and surface asperities have been previously observed when attempting to incorporate Al3+ dopants into low-temperature synthesized ZnO nanorods and can be explained by this same theory.11 Finally, the effect of cations finding unfavorable bonding environments on the overall nanocrystal shape is more disruptive early in the growth process (when the

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nanocrystal shape is still developing) compared to more mature stages during the reaction when the nanocrystals have achieved their near-final morphology.

Figure 2: TEM Images of ZnO nanorods doped with different strategies. Undoped nanorod controls (A), 5% In – doping throughout synthesis (B), 3% In – doping after seeds form (C), 3% In – ‘core/shell’ doping (D), 3% Al – doping after seeds form (E), 3% Al – ‘core/shell’ doping (F)

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The nanorods grown using undoped seeds as templates (Figure 2C/E) exhibit a noticeably improved morphology relative to those synthesized with the dopant present after the initial hot injection (Figure 2B).

By growing undoped nanorod seeds as

morphological templates, the nanocrystal shape is preserved for the most critical step – the nucleation of relatively monodisperse, isotropic nuclei that will serve as the basis for the elongation process during the growth reaction. While it appears that for both Al- and In-doped nanorods that some surface inhomogeneities still develop during the dopant incorporation process, the undoped nanorod templates clearly serve as a basis for more uniformly shaped rod-like particles. The preserved morphology is also clearly observable in the ‘core/shell’ type doped ZnO nanorods (Figure 2D/F), and is similarly observed for higher dopant concentrations (5% Al or In; SI, Figure S5). TEM images of the in-situ post-growth doping (Figure 1B) nanorod samples show similar morphologies to the undoped nanorod controls. (SI, Figure S5).

Structural Proof of Dopant Incorporation Dopant incorporation and nanocrystal morphology can also be studied using XRD. The XRD data shown in Figure 3 for In:ZnO samples doped throughout the synthesis (Figure 1A) and Al:ZnO ‘core/shell’ samples (Figure 1D) show pure wurtzite ZnO with no detectable presence of secondary crystalline phases such as In2O3 or Al2O3. The anisotropic shape of the nanorods is confirmed by comparing the relatively smaller peak breadths of the (002) peaks, corresponding to the length of nanorod, with the broader (100), (101) and other radial peaks, corresponding to the width of the nanorod, of the diffraction pattern. The normalized versions of the XRD data show shifts in peak

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position that track with the dopant cation size. Figure 3B shows a normalized inset of the (002) peak, which clearly indicates a shift in the peak position to smaller 2θ in the case of indium doping, corresponding to an increase in lattice spacing, and a shift in peak position to larger 2θ, indicated a decrease in lattice spacing, in the case of aluminum doping (lattice parameters from Rietveld analysis are shown in the SI, Figure S3). This effect is similarly observed in the case of the (101) peak, shown in Figure 3C. This trend is indicative of substitutional doping, where a zinc atom is replaced by a dopant atom, rather than interstitial doping, where a dopant atom is incorporated in interstitial sites in the lattice, and can be understood by the larger size of indium and smaller size of aluminum relative to zinc. Furthermore, this result confirms the successful incorporation of dopants in the lattice when dopants are introduced using either the continuous doping or core/shell methods, despite the poor morphological control the former affords. Surface segregated dopants, for example, may not contribute to the overall free electron population due to clustering, secondary phase nucleation, or compensation by surfacebound charges. As a result, the peak shifts indicative of substitutional doping are not observed in the case of the in-situ post-growth doping (Figure 1B), shown in the Supporting Information, Figure S1.

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Figure 3: Powder X-ray diffraction scans of doped ZnO nanorods. XRD scans of Indoped, Al-doped, and undoped ZnO nanorods (A). Black lines indicate positions of wurtzite ZnO peaks (JCPDS #36-1451). Normalized data is shown for the (002) peaks (B) and (101) peaks (C), indicating peak shifts corresponding with dopant ionic radius size.

Elemental Analysis of Dopant Incorporation A key indicator of dopant activation is the type of atomic site the dopant occupies in the nanocrystal host lattice. Thus, XPS was used as a tool to analyze the relative

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amount of surface indium as a function of time during growth of the undoped shell after doping with 3% In precursor solution (Figure 1D); using WANDA, aliquots were taken over time during the undoped zinc oxide shell growth reaction. As the number of Zn injection sets increases, the In surface concentration decreases due to growth of an undoped ZnO shell (Figure 4). The dopant concentration in the syntheses, determined by reactant ratios, and the surface indium concentrations, determined by XPS, are shown in Table 1. Al signal in the ZnO nanorods was not resolvable by XPS. This is likely due to low Al dopant concentration – on the order of 1-2% (at.) as determined by ICP (SI, Table S1) - and the relative sensitivity factor of Al 2p peaks being almost an order of magnitude less than that of the In 3d peaks.31

Figure 4: XPS characterization of In-doped ZnO nanorods.

Indium dopant

incorporation is measured over time during undoped shell Zn injection sets (‘core/shell’ type synthesis). Area ratios of the In 3d3/2/3d5/2 and Zn 2p1/2/2p3/2 peaks indicate a decrease in the relative amount of surface In with increasing shell injections as In gets incorporated into the ZnO shell (A), and normalized XPS spectra of the In and Zn regions for increasing numbers of shell injection sets (B). XPS spectra shown in (B) were normalized to the Zn 2p3/2 peak area.

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Reference Figure 1A doping throughout synthesis Figure 1B in-situ post-growth doping Figure 1C Doping after seeds form Figure 1D ‘Core/Shell’ type doping Figure 1C Doping after seeds form Figure 1D ‘Core/Shell’ type doping

Doped Injection Set Concentration (mol%)

Overall Dopant Concentration (mol %)

Surface Indium Concentration (XPS) (mol%)

In (5%)

4.2

69

In (5%)

2.1

67

In (3%)

0.9

14.7

In (3%)

1.27

7.4

Al (3%)

0.9

not resolvable

Al (3%)

1.27

not resolvable

Table 1: ZnO nanorod dopant characterization. Dopant concentrations in synthesis (during injection sets/overall) determined by reactant ratios and surface In concentration measured using surface elemental analysis (XPS). The indication of 3% and 5% dopant refer to the relative molar concentration of the dopant precursor during the injection sets where dopant addition is taking place (shown schematically in Figure 1). Indium surface concentrations were calculated by adjusting for the relative sensitivity of the XPS instrument to In/Zn (described in the Supporting Information).

The difference in doping efficiency between Al and In (Table 1) can be explained by the difference in relative reactivities of the Al and In acetate precursor salts, which is related to the difference in stability between the precursor salts and the final product (Al:ZnO or In:ZnO). By Pearson’s Hard-Soft Acid-Base (HSAB) theory, stronger interactions occur between acids/bases of similar hardness – harder acids/bases typically exhibit low polarizabilities, larger charge-to-radius ratios, and higher charge states.32 Thus, acetate anions are considered to be harder bases owing to the relatively high degree of negative charge localization. Similarly, Al3+ cations are harder acids than In3+ cations

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due to the smaller ionic radii of Al and increased charge to radius ratio. The interaction between acid and base entities of similar degrees of hardness results in a more stable bond and, in the case of doping nanocrystals, a less reactive dopant precursor. For Al doping of ZnO nanorods using acetate salts, the increased relative stability of aluminum acetate relative to indium acetate results in a lower Al incorporation efficiency. Despite the different In/Al dopant precursor reactivities, the choice of acetate salts was made to keep the anion chemistries present in the reaction system the same, thereby preventing differences due to detrimental anion-induced morphology effects that have been observed in various reaction systems.8,11,33-35 In addition to influencing dopant incorporation, the use of different dopant salts also affects the shape of the nanorods. Figure 5 shows size distribution data for the lengths and diameters of the nanorods doped with 3% In and Al. While nearly independent control of shape and doping is exhibited using this strategy, some subtle differences in the nanorod shapes are observed with In and Al dopants. ZnO nanorods doped with In are shorter in length and exhibit larger diameters relative to rods doped with Al. Aluminum doping, on the other hand, results in nanorods with larger aspect ratios. Furthermore, upon closer examination of the (101) peak inset shown in Figure 3C, a smaller peak breadth for indium doped ZnO nanorods and correspondingly larger radii compared to undoped Al-doped nanorods is observed - implying that the choice of dopant can alter the nanorod aspect ratio. This particular example of dopant-induced shape effects has not been previously observed in colloidal zinc oxide systems and could be an alternative strategy to control nanorod morphology and dopant incorporation. One possible explanation is that differing

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equilibria between the dopant cations and phosphonic acids could affect the amount of free surfactant in the reaction mixture. The free surfactant concentration is a critical variable in controlling nanocrystal dimension – the surfactant competes with reactive monomers for binding sites on the nanocrystal surface during the growth process. Higher phosphonic acid surfactant concentrations in this reaction mixture result in the growth of smaller diameter, higher aspect ratio nanorods due to more effective surface capping.16 Alternatively, the more polarizable, larger In3+ dopant cations can create strained bonding environments with high concentrations of surface defects upon adhering onto the nanorod surface, This possible tendency of In3+ sites to create a defective bonding environment can promote amorphous growth around areas poorly passivated by the surfactants in solution. Further, this theory could also help explain why larger indium and gallium cations are commonly used as dopants to grow amorphous zinc oxides while aluminum is more commonly used as a dopant to grow crystalline doped zinc oxide.36,37

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Figure 5: Nanorod size distributions as a function of time for ‘core-shell’ type doping. Evolution of In/Al-doped ZnO nanorod length distributions with time (A), evolution of diameter distributions with time (B), and schematic of the doped nanorod nucleation and growth process (C).

Optical Characterization of doped ZnO Nanorods The most common heteroatoms used to dope ZnO are Al, Ga, In, and B – all Group III elements that can act as donor impurities when sitting substitutionally on Zn sites. Aliovalent doping is achieved in ZnO by incorporating these impurity atoms into the crystal structure of the intrinsic ZnO semiconductor. Thus, a key metric of dopant activation is not number of incorporated dopant atoms, but rather the number of dopant

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atoms that contribute to the host material free carrier density. Thermodynamic treatments of doping nanocrystals involve more than the defect equilibria typically derived from bulk material compositions – at the nanoscale, for example, compensation of free carriers by surface defects and bound ligands/molecules can result in drastic changes to the free carrier concentrations since nanocrystals often have only a small number of free carriers per crystal9. For example, a typical doped ZnO nanorod from this work with a free carrier concentration of 1021 cm-3 would have ~200 free carriers per nanocrystal. Compensation of a single free electron by a surface entity or defect thus would have a non-trivial effect on the overall free carrier concentration in nanocrystals and resulting optoelectronic properties. The strategies of doping the nanorods after seeds form and ‘core/shell’ doping (Figure 1D) enable both Al and In dopants to be incorporated into the nanorod host lattices. The changes in the optoelectronic behavior upon dopant incorporation can be characterized by the Burstein-Moss shift of the absorption edge and the near-infrared (NIR) localized surface plasmon resonance (LSPR) line shape. The Burstein-Moss shift is a measureable increase in the optical band gap of the material with doping owing to the increased occupation of conduction band states by free electrons, and commonly used to characterize the optical band gap of metal oxide thin films and nanocrystals.10,13,33,38,39 Optoelectronic characterization of the nanorods doped with 3% Al and In solution using the strategies outlined in Figure 1 is shown in Figure 6 (data for the 5% Al or In doping samples are located in the SI).

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Figure 6: Optical characterization of doped zinc oxide nanorods. Plots show UV-vis spectra, insets show band gap estimation from the Burstein-Moss effect. Doping after seeds form and ‘Core/Shell’ type doping series with 3% In doping (A), 3% Al doping (B) All spectra are normalized to undoped ZnO band edge absorption (green).

For both In and Al dopants at 3% and 5% doping concentrations, the optical band gap and NIR free carrier absorption features follow similar trends (shown in Figure 6, Figure S4, and Table S2). Relative to the absorption behavior of undoped nanorods, the nanorods doped with In or Al after undoped seed formation demonstrate increased free carrier absorption as well as an increase in the optical band gap. Upon growth of the undoped ZnO shell during the ‘Core/Shell’ doping process for both Al- and In dopants (Figure 1D), the NIR absorption of the doped nanorods increases dramatically relative to the nanorods doped after seed formation (Figure 1C), thus indicating an increase in the incorporation and activation of the dopant cations during the shell growth step. The optical band gap estimates from Tauc Plots (insets, Figure 6) show the Burstein-Moss shift with increased doping – the optical band gap increases due to dopant-generated free carriers occupying available states in the conduction band. While it is possible that the band gap shifts observed with doping are due to changes in nanocrystal electronic

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structure, this would likely be a secondary effect due to the low dopant concentration (< 1.5% at. by ICP, see SI Table S1). For both dopant concentrations tested (3% and 5%), the In:ZnO nanorods result in more pronounced changes in the optoelectronic properties than Al:ZnO nanorods as well as higher nanocrystal dopant concentrations measured by elemental analysis. Finally, the optical properties of the nanorods doped after seed formation and subsequently ‘core/shell’ doped demonstrate that dopant incorporation is enabled by the shell growth process and results in an increase in the free carrier concentration of these materials.

Characterization of Doped Nanorod Plasmonic Properties The plasmonic behavior of these doped nanorods is characterized by measuring the infrared response of the nanocrystals. As shown in Figure 6, a pronounced NIR absorption feature appears upon dopant introduction and subsequent shell growth. This feature is associated with an increase in free electron density upon incorporation and activation of extrinsic dopants, which affects the position of the LSPR – typically for ZnO, it is observed to be between 2.5-10 µm, depending on the doping level.8,38,40 The plasmon resonance frequency depends on the square root of the free carrier concentration

(ω

P

)

∝ n and increases with increasing dopant levels (if the dopants are activated).40,41

The LSPR absorption for samples doped with increasing amounts of indium (measured by ICP) is shown by FTIR measurements in Figure 7. As the concentration of dopants in the nanocrystal increases, the broad LSPR absorption feature progressively blueshifts to higher wavenumbers indicating a larger number of free carriers present in the nanocrystal. For calibration, isotropic doped ZnO nanoparticles with an assumed meff of

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0.257,42 exhibit a LSPR of 2000 cm-1, which corresponds roughly to free carrier densities in the range of 91019 - 1020 cm-3 (see SI for details of calculation).13,38,43 While splitting of the LSPR modes due to shape anisotropy has been observed in other nanocrystal systems,44 in this case a single broad absorption feature is observed for the doped ZnO nanorods. It is important to note that the correlation between the peak position and doping levels is only semi-quantitative, since the peak shape and location can also affected by a variety of other factors, including nanocrystal agglomeration, polydispersity, and nanocrystal shape.40,41,45

Figure 7: FTIR spectra in transmission for anisotropic ZnO nanocrystals. FTIR of ZnO nanorods with increasing levels of indium doping (A) and normalized UV-vis/FTIR spectra comparing the plasmonic behavior of 3% Al doped and 3% In doped ‘core/shell’ type syntheses (B). Dotted lines in (A) are guides to the eye. UV-vis and FTIR are joined at 2500 nm = 4000 cm-1. Differences observed in the fingerprint region can be attributed to incomplete purification of the nanocrystals after synthesis – varying amounts of residual surfactants could be present in the different samples measured by FTIR.

The broadband optical response (transmission UV-Vis-NIR and FTIR spectra) of the nanocrystals can be a useful tool to study the free carrier behavior in nanocrystals doped using different strategies. Figure 7B shows the plasmonic behavior of the doped

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ZnO nanorods and a blueshift of the LSPR absorption onset for indium-doped ZnO nanorods relative to aluminum-doped nanorods. This blueshift corresponds to a higher free carrier concentration in indium-doped nanorods. Further, this observation is consistent with the hypothesis that a higher reactivity of the indium dopant results in increased dopant incorporation relative to aluminum doped nanorods and with the optical behavior shown in Figure 6. The carrier concentrations estimated from the LSPR peak positions correspond roughly to free carrier concentrations on the order of 1020 cm-3, which are comparable to the highest carrier concentrations achieved by vacuum deposited doped ZnO films, which are typically processed at much higher (> 600 °C) temperatures. The level of dopant incorporation observed here is an indication that this doping strategy is highly efficient in generating free carriers, despite the kinetic and morphological limitations imposed by low-temperature colloidal syntheses. Lounis et al. found that dopant distribution strongly affected the plasmon absorption peak shape and line widths in isotropic ITO colloidal nanocrystals.41 The undoped core, doped shell structure creates an environment where free carriers are minimally perturbed by dopant-induced defects and as a result exhibit more symmetric peak shapes with less damping. Similarly, we hypothesize that some of these effects on the plasmon shape are similarly occurring with the doped zinc oxide nanorods. Further characterization of the plasmonic characteristics of these nanocrystals could provide insight to the nature of the free carrier absorption behavior in these anisotropic nanocrystals.

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CONCLUSION In summary, a strategy for independent shape and doping control of colloidal zinc oxide nanorods was presented. This method utilizes undoped ZnO nanorod seeds as morphological templates for the synthesis of doped nanocrystals, and grows a ZnO shell over the nanorods to incorporate surface segregated dopants and overcome kinetic limitations associated with synthesizing oxide nanocrystals at low temperatures. This approach is shown to be compatible with multiple dopant cations, indicating the possibility of extending this methodology to other dopants and materials. Simultaneous control of shape and doping in colloidal zinc oxide nanorods has not been previously demonstrated, and these results further show that differences in dopant cations result in subtle differences in the reaction chemistry that enable nanostructure aspect ratios tuned by dopant selection. The ability of indium to induce nanorod growth with smaller aspect ratios and high dopant incorporation efficiencies could be used as a strategy to direct growth of specific morphologies for applications in sensing or optoelectronics.

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SUPPORTING INFORMATION AVAILABLE: Further characterization data (TEM, XRD, optical spectroscopy, and elemental analysis) of the samples discussed in this manuscript and detailed calculation methods (XPS surface analysis and carrier concentrations from plasmon absorption) are available in the Supporting Information.

ACKNOWLEDGEMENTS: The authors gratefully acknowledge the National Science Foundation Award No. DMR1007886 for financial support. All experiments were performed as part of the Molecular Foundry User Program, supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC0205CH11231. Ms. Bergerud was supported by a National Science Foundation Graduate Research Fellowship under Grant No. DGE1106400.

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Shell Approach to Dopant Incorporation and Shape Control in

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