InP Nanoflag Growth from a Nanowire Template by in Situ Catalyst

Letter pubs.acs.org/NanoLett

InP Nanoflag Growth from a Nanowire Template by in Situ Catalyst Manipulation Alexander Kelrich,*,† Ofir Sorias,† Yonatan Calahorra,†,§ Yaron Kauffmann,‡ Ran Gladstone,† Shimon Cohen,† Meir Orenstein,† and Dan Ritter*,† †

Electrical Engineering Faculty and ‡Department of Materials Science and Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel § Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, United Kingdom S Supporting Information *

ABSTRACT: Quasi-two-dimensional semiconductor materials are desirable for electronic, photonic, and energy conversion applications as well as fundamental science. We report on the synthesis of indium phosphide flaglike nanostructures by epitaxial growth on a nanowire template at 95% yield. The technique is based on in situ catalyst unpinning from the top of the nanowire and its induced migration along the nanowire sidewall. Investigation of the mechanism responsible for catalyst movement shows that its final position is determined by the structural defect density along the nanowire. The crystal structure of the “flagpole” nanowire is epitaxially transferred to the nanoflag. Pure wurtzite InP nanomembranes with just a single stacking fault originating from the defect in the flagpole that pinned the catalyst were obtained. Optical characterization shows efficient highly polarized photoluminescence at room temperature from a single nanoflag with up to 90% degree of linear polarization. Electric field intensity enhancement of the incident light was calculated to be 57, concentrated at the nanoflag tip. The presented growth method is general and thus can be employed for achieving similar nanostructures in other III−V semiconductor material systems with potential applications in active nanophotonics. KEYWORDS: InP, nanoflag, nanomembrane, catalyst unpinning, selective area VLS growth, linear polarization, field enhancement

T

implemented to synthesize high crystal purity InAs nanoplates by vapor−solid growth on high-energy facets.19 Here, we demonstrate a highly controllable new method for the growth of 2D structures on the side facets of nanowires by an in situ manipulation of the catalyst. In the first part of this work, we describe the process by which the catalyst can be displaced in situ from the nanowire top to its sidewall. Then we describe the growth of flag-like nanostructure perpendicular to the primary nanowire axis by a catalyst assisted process at predefined position along the nanowire. Finally, we report on the optical characterization of the nanoflags by employing polarization resolved photoluminescence (PL) and field enhancement simulations. The primary nanowire growth was carried out on (111)B InP substrates by the selective-area VLS20 technique, see Methods for details. In brief, 0.5−2.5 μm long nanowires were grown from gold catalysts having a diameter of 25−50 nm that were precisely positioned into a SiNx mask layer openings. The catalyst in situ unpinning is schematically described in Figure 1, panel a by stages I−III. During the VLS growth, the catalyst was positioned on the (000−1) pedestal at the top of the nanowire.

he inherent one-dimensionality of semiconductor nanowires has stimulated extensive research on their synthesis1−3 and applications.4−9 Compound III−V semiconductor nanowires are often obtained via the vapor−liquid−solid (VLS)10 or vapor−solid−solid (VSS)11 mechanisms, where supersaturation of dissolved vapor-phase precursors in liquid or solid metallic nanoparticle drives precipitation of the crystalline layer at the interface between the growing nanowire and the catalyst. A unique feature of the VLS method is that different crystal structures of the same material system can be obtained.12 Pure zinc blende (ZB) to pure wurtzite (WZ) transitions were demonstrated by the control of growth parameters such as temperature and V/III flux ratio in InAs, GaAs, and InP nanowires.13,14 Concurrently with the progress in nanowire research, an increasing interest in semiconductor nanomembrane synthesis and applications15 is immerging. Top-down nanomembrane fabrication such as exfoliation and anisotropic etching is the common approach,15 but bottom-up epitaxial growth of 2D nanostructures has also been demonstrated. Recent examples are selective area growth of defect free GaAs nanosheets,16 InAs wing-shaped nanomebranes on Si,17 and twin-induced InSb nanosails.18 An intriguing bottom-up approach for achieving free-standing, semiconductor 2D nanostructures is offered by the epitaxial growth on the sidewall of nanowires. This technique was © XXXX American Chemical Society

Received: February 14, 2016

A

DOI: 10.1021/acs.nanolett.6b00648 Nano Lett. XXXX, XXX, XXX−XXX

Letter

Nano Letters

Figure 1. (a) Schematic illustration of nanoflag and single branch nanowire growth by controlled in situ catalyst displacement. Stage I: Au droplet is positioned inside SiNx mask opening by e-beam lithography. Stage II: growth of primary nanowire. Stage III: the catalyst is detached from the top pedestal and migrates along one of the side facet. Stage IV: growth of a nanoflag or a branch nanowire is initiated. (b) Diagram of the growth parameters that determine the transition from primary nanowire growth to catalyst displacement, shown in stages II and III in panel a. The TMI flow rate marked in green sets a planar growth rate of 110 nm h−1 on an InP(001) substrate at 500 °C.

Figure 2. (a) SEM image viewed at 30° of an InP nanowire array following catalyst unpinning. The nanowires are approximately 1.5 μm long and 25 nm wide. The inset shows 45 nm wide nanowire, highlighting the correlation between catalyst position and sidewall truncation. (b) HAADF-STEM image demonstrating typical nanowire morphologies after catalyst unpinning. The bright points indicate gold particle location. (c) HR-TEM image of truncated nanowire tip previously occupying the catalyst. (d) HR-TEM image of the Au particle catalyzing sidewall regrowth during its migration along the same nanowire, shown in panel c. The inset shows schematically the primary nanowire facets in red and the overgrowth in green. Catalyst− nanowire interface plane is indicated. (e) HR-TEM image of nanoflag nucleation during the unpinning procedure. The HR-TEM images were taken along zone axis.

The in situ displacement procedure results in positioning of the catalyst on one of the six equivalent {1−100} side facets in a controlled manner. As specified in Figure 1, panel b, upon reaching the desired nanowire length, the trimethylindium (TMI) supply was turned off while the temperature was raised from the growth temperature of 420 to 515 °C. The PH3 flux was reduced by a factor of three but not switched off completely to prevent decomposition of the InP nanowires. These conditions were maintained for 5 min. To monitor the catalyst displacement procedure, we have examined by scanning electron microscopy (SEM) a nanowire array that was removed from the growth chamber before subsequent nanostructure growth as shown in Figure 2, panel a. About 40% of the catalysts migrated 300−500 nm toward the base of the nanowire, 40% have nucleated triangular growth perpendicular to the nanowire axis adjacent to the nanowire tip, and the rest migrated a few hundred nanometers and then initiated triangular growth. The high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image shown in Figure 2, panel b provides better resolution of the two typical cases.

Some insight on the catalyst unpinning process from the (000−1) pedestal at the top of the nanowire can be obtained from the high-resolution TEM (HR-TEM) image shown in Figure 2, panel c, where the pedestal transformed into an asymmetrically truncated and rounded tip. Additional HR-TEM images of wider nanowire tips showing that the newly emerged (one or more) facets form {1−10−1} planes are provided in the Supporting Figure S1. Oblique (1−11) facet formation was previously observed by in situ TEM of Si nanowire kinking, originally growing in the [1−1−1] direction.21 There, the catalyst wetting of the nanowire sidewall occurred through formation of an oblique facet until an equilibrium “pivot” point was reached, followed by nanowire growth in [1−11] direction. Recent in situ growth experiments have demonstrated that a truncated morphology at the growth interface occurs in a many material systems including III−V semiconductors22−24 as nanowire growth proceeds by periodic growth-dissolution B


Letter

Nano Letters

Figure 3. (a, b) SEM images of an InP nanoflag array. (a) Top view. The inset shows magnified single structure demonstrating that the flag is growing perpendicular to the nanowire sidewall. (b) Tilted view at 30°, demonstrating different nanoflag geometries. (c) Schematic of nanoflag growth direction distribution among six {11̅00} sidewalls of the flagpole, based on 700 nanoflag sample shown in the Supporting Figure S3. (d−f) SEM images viewed at 30° of top, middle, and bottom flag geometries, respectively.

the growth conditions. Spontaneous Au catalyst movement to the InGaP nanowire {111}B sidewall during sample cool down was recently reported.36 The particle movement was, however, accompanied by material removal along its 100−200 nm path rather than regrowth, as in our case. Catalyst migration along the nanowire requires a defect-free structure as shown in Figure 2, panels c and d and further demonstrated below. In other words, a few (or sometimes even a single) stacking faults can pin the catalyst and prevent its migration. Stacking faults at the nanowire−catalyst interface can act as preferred nucleation location, pinning the triple-phase boundary.36,37 At the top of longer nanowires (Figure 2a), having high crystal defect density, the catalyst does not migrate downward, and instead triangular growth takes place immediately after catalyst displacement, as shown in Figure 2, panel e. The top view of these structures is shown in the Supporting Figure S2. The volumes of the growth on the facets during catalyst migration and the triangular structure nucleation are similar, indicating comparable indium collection and utilization by the catalyst despite distinct growth processes. We now proceed to describe the catalyst assisted growth of nanomembranes following catalyst migration along the nanowire. After the catalyst unpinning procedure, the sample was maintained at 515 °C, and the same TMI and PH3 flow rates were resumed as during the growth of the primary nanowires. Figure 3, panels a and b show the SEM images of planar and 30° tilted views of the obtained arrays after 40 min of growth, comprising flag-like nanostructures. The length of these “nanoflags” was about 1 μm and their thickness about 40 nm, matching the nanowire diameter, with slight tapering toward the flag tip. The flags grew perpendicularly to the six equivalent {1−100} planes constituting the nanowire facets, as shown in the inset of Figure 3, panel a, with a random distribution as schematically illustrated in Figure 1, panel c, based on the analysis of 700 structures shown in Supporting Figure S3. The nanoflags can be categorized into three types related to their

cycles. Every period starts by growth on a newly formed oblique facet accompanied by an increase in catalyst supersaturation, which lowers the kinetic barrier for nucleation. Then step flow growth takes place on the main (000−1) facet, leading to supersaturation drop and dissolution of material in the truncated area, completing the growth cycle.24 In our case, since the TMI supply is aborted during the heating step, no growth takes place on the oblique {1−10−1} and main (000− 1) facets. As a result, because of the higher temperature and the lower P2 flux, supersatuaration is reduced,25 providing the driving force for successive dissolution events that create the large truncated area. Because of the incorporation of additional indium atoms from the dissolved area into the catalyst, its volume increases, eventually causing catalyst transition to the truncated sidewall, as emphasized in the inset of Figure 2, panel a. Previous theoretical studies have indeed attributed the catalyst unpinning process to expansion of the liquid particle, combined with a reduction of the nanowire base area.26−28 Interestingly, as the gold particle migrates, catalyst mediated crystal growth takes place on the sidewall, as shown in Figure 2, panel d. Since no TMI was supplied during this step, the source of indium must be the nanowire itself undergoing thermal decomposition of its external surfaces. Controlled contraction of nanowire diameter and length at elevated temperatures was recently demonstrated for self-catalyzed GaAs29 and Au catalyzed InAs nanowires.30 The higher temperature and reduced P2 flux during catalyst migration result in phosphorus desorption from the nanowire facets, while the remaining indium atoms diffuse toward the catalyst. Figure 2, panel d shows that the catalyst is aligned along the (−1101) plane, which is probably a favored growth plane due to its high atomic density.31,32 A somewhat similar phenomenon of nanowire regrowth was previously achieved by the insertion of a physical perturbation leading to catalyst transition from the top facet to nanowire sidewall.33−35 In our case, however, nanowire dissolution and regrowth were achieved solely by variation of C


Letter

Nano Letters

Figure 4. (a) TEM image of an InP nanoflag at the top of the nanowire. (b) HR-TEM image of the nanostructure shown in panel a demonstrating stacking faults defects transferred from the nanowire to the nanoflag, confirming the epitaxial relationship between them. The inset shows the fast Fourier transform (FFT) proving the WZ structure of the flag. (c−f) HR-TEM images of different magnified areas of the nanoflag. The rim color in each figure matches the colored areas in panels a and b, which were magnified. (g) TEM image of a flag at the middle of the nanowire. The arrow shows the direction of growth. (h) HR-TEM image of the nanoflag tip indicated by yellow square in panel g. A single crystal defect in the flag is marked by an arrow. Typical crystallographic planes are marked. The crystal structure of the flag is pure WZ as confirmed by FFT in the inset. (i) Another pure WZ middle flag demonstrating a single stacking fault pinning the catalyst. All HR-TEM figures are taken along {11−20} zone axis.

between wire and flag. As a result, the nanoflag crystal structure is WZ, as illustrated by the fast Fourier transform (FFT) in Figure 4, panel b. Structural defects in the nanowire are transferred to the flag area, appearing as planar defects perpendicular to the flagpole, some of them extending all the way to the catalyst as shown in Figure 4, panel c. Two sidewalls of the nanoflag are defined by {11−20} planes, as opposed to lower surface energy {1−100} facets38 of the nanowire. Additional exposed facets are shown in Figure 4, panels b−f. The triangular shape is defined by two {1−104} planes creating an angle of 50.5° at the tip of the flag and a {1−100} nanowire facet constituting the third edge. The transition between flag and pole at the bottom vertex is composed of a short section of {1−102} planes as illustrated in Figure 4, panel e. The upper area of the top flag is bordered by the (000−1) plane (same as the growth front of the original nanowire) that evolves when an upper flag inclined facet reaches the nanowire tip. In the flags located at the bottom of the nanowire, the base facet is constrained during growth by the SiNx layer on the substrate and transforms to a {0001} plane, leading to right triangle flag growth directly on top of the mask layer. In Figure 4, panels g−i, we observe pure WZ nanoflags with just a single stacking fault originating in the otherwise pure WZ nanowire template. A stacking fault in the nanowire is required to pin the catalyst during its migration along the side facet. It is

position along the nanowire “flagpole” and their geometry, as shown in Figure 3, panels d−f. The shape of the nanoflags at the nanowire top is quadrilateral, the middle flags grow as isosceles triangles, and the bottom flags have right triangle geometry, bordered by the nanowire and the substrate. In all three flag types, the catalyst is located at the tip of the nanoflag. The thickness of the nanoflags is determined by the width of the nanowire, which in turn varies along the nanowire flagpole due to tapering. Thus, nanoflags at the bottom of the flagpole are thicker than the ones at the top. Thinner nanoflags in the sub 20 nm range were also successfully grown (see Supporting Figure S4). The identification of the nanoflag crystal structure and facets was performed by HRTEM. The nanoflags were mechanically removed from the substrate, transferred to a holey carbon grid, and analyzed using an aberration corrected Titan 80−300 FEG-S/TEM (FEI) operated at 300 keV. Representative low-resolution TEM images of top and middle nanoflags are shown in Figure 4, panels a and g, respectively. Overall, ten nanoflags of each type were examined. The nanowire flagpoles were grown at conditions (see Methods) favoring WZ crystal structure with increasing probability for stacking fault defects at higher growth rates. Since growth rate increases with nanowire length, more defects are found at the edge of the nanowire.25 The TEM pictures clearly show that the nanowire is a template for nanoflag formation since no interface or defect is observed D


Letter

Nano Letters further shown below that controlling the position of the defects along the nanowire allows control of the flag position. The demonstration of an InP nanomembrane having pure WZ crystal structure (with a single stacking fault) is a notable result of this work. The nanoflags are an interesting example of catalyst induced crystal growth, which promotes additional well controlled nonVLS growth. The evidence for this unusual crystal growth mechanism is obtained from the observed position of the catalyst, which is not located exactly at the tip of the flag but is displaced toward the (−1 1 0 4) facet of the flag (toward the base) as shown in Figure 4, panel h. The interface between the catalyst and the nanoflag is of the same crystallographic {1− 101} orientation as during catalyst migration along the side facet, as shown in Figure 2, panel d. Hence, the catalyst mediated VLS growth occurs along the same energetically favorable planes during both the flag growth process and the migration process (which is also accompanied by overgrowth, as discussed earlier). The shape of the nanoflags thus indicates that they grow by a combination of VLS and direct nucleation on the inclined {1−10−4} facets, as atomic planes created at the catalyst-nanoflag interface immediately induce growth on inclined facets to complete the atomically flat surface. A related example of catalyst induced radial growth is shown in the Supporting Figure S5. As mentioned earlier, a single crystal defect in the nanowire may pin the Au particle, yet the catalyst may in some cases leap across several stacking faults before anchoring, as can be seen in Figure 4, panel b. This observation opens the way for a planned positioning of the nanoflag along its pole by controlling the stacking fault density. The strategy to achieve this goal is based upon the introduction of defects into the primary nanowire by controlling the chemical potential inside the catalyst through variation of P2 flux.25 To test this procedure, we have grown three types of nanowire “poles”. The first sample consisted of 3−4 μm long nanowires with high defect concentration at the top due to the increasing growth rate of longer nanowires. Next, we have obtained pure WZ nanowires by increasing the phosphine supply from 3 to 5 sccm. The third sample comprised similar pure WZ nanowires, but in the middle of each nanowire we have inserted a 150 nm long segment with high defect density by reduction of the phosphine flow rate to 1 sccm. All three samples proceeded to nanoflag growth, which resulted in different geometries shown in Figure 5. Nanoflags grown on long flagpoles were all pinned to the nanowire tip resulting in top flag geometry (Figure 5a). The middle flags were successfully grown at high yield close to 100% on the flagpoles containing the high defect density segment (Figure 5b). No catalyst migrated beyond the segment. An HAADAF-STEM image shown in Figure 5, panel d of a single middle flag emphasizes catalyst location in the defective segment. In an additional experiment (not shown), we have found that when the length of the mixed WZ-ZB section was reduced to 50 nm, about 10% of the catalysts migrated beyond the segment. The transition between high defect density section to the pure WZ phase section is shown in Figure 5, panel e. Lattice image of the transition area is shown in Supporting Figure S6. Finally, pure WZ flagpoles did not pin the catalyst, resulting in dominance of bottom flag shape (Figure 5c). The bulges seen in Figure 5, panel c are not related to defects formation and were caused by incomplete catalyst unpinning.

Figure 5. Controlling the flag location along the nanowire by the intentional incorporation of defects. (a−c) SEM images of InP nanoflag arrays viewed at 30°. (a) Long InP nanowires with inherent high defect density in the top area of the flagpole, which pins the catalyst, resulting in top flag geometry. (b) Middle nanoflags produced by intentional defect creation in the middle section of the flagpole. (c) Bottom flags created by growing defect free primary nanowires under high phosphine flow. In this case, the catalyst migrates to the nanowire base resulting in bottom flag geometry. The bulges seen on top of some of the nanowires are a consequence of nonoptimal unpinning procedure. The scale bar is 2 μm unless mentioned otherwise. (d) HAADF-STEM image of a single InP middle flag demonstrating the catalyst is anchored to the polytypic segment. (e) HR-TEM image demonstrating the transitions between the polytypic segment and pure WZ parts of the nanoflag. The arrow indicates the growth direction of the flagpole.

As discussed earlier, the unpinning process on defect free nanowires may result in catalyst migration all the way to the substrate. In this case, suitable growth conditions will lead to planar nanowire growth right on top of the amorphous mask covering the substrate as schematically shown in Figure 1, panel a. Growth and characterization of planar nanowires will be discussed in detail elsewhere. In the last part of this publication, we describe the optical properties of the obtained nanoflags. Optically active nanostructures are of a great interest for a range of applications.39−41 It is well-known that III−V semiconductor nanowires exhibit polarized PL depending on their geometry and crystal structure.42−44 Hence, we studied the polarization dependence of the nanoflag absorption and emission. Additionally, we demonstrate by simulations a significant field intensity enhancement at the nanoflag tip. Polarization resolved PL measurements at room temperature on individual nanoflags were performed with a microphotoluminescence setup, see Supporting Figure S7 and Methods for details. A linearly polarized laser was focused to ∼4 μm spot, comprising several nanoflags within a 2 μm pitch array. The laser excites mainly the nanoflags with elongation axis coinciding with laser polarization (see Supporting Figures S8 and S9). In this way, it was possible to excite and measure E


Letter

Nano Letters

Figure 6. (a) Camera picture showing PL from a single nanoflag in a 2 μm pitch array. (b) Polarization resolved PL intensity, demonstrating polarized emission from a single WZ nanoflag, along with weaker, unpolarized emission from the ZB substrate. (c) Polar plot of the normalized nanoflag emission at 860 nm (green circles) and substrate emission at 925 nm (black circles) with 74% and 23% degree of polarization, respectively. (d, e) Normalized polarization-resolved PL intensity of other two typically orientated nanoflags, shown in the inset of each plot. The scale bar is 500 nm. In all polar plots, the circles show measured data points, and dashed lines represent sin2 fits to the data. (f) |E|2 field enhancement profile along nanoflag cross-section. Logarothmic color bar presented on the right shows maximum field intensity enhancement of 57. The profile is taken at wavelength of 1350 nm.

μm pitch array comprised nanoflags with a 2 μm long nanowire pole with a diameter of 60 nm and a 0.8 μm long flag. The maximum field intensity enhancement at the tip was 57 at a wavelength of 1350 nm, as shown in Figure 6, panel f. This strong local field enhancement includes also a contribution from a standing wave pattern formed due to substrate reflection, constructively interfering with tip localized fields. The large field enhancement at the nanoflag tip, and the low absorption of InP at the telecommunication wavelengths, renders them efficient nanoantennas for light emitters located at the tip.45 Furthermore, nonlinear effects such as second harmonic generation can be intensified for this type of tapered nanostructures, as was shown for InAs V-shaped nanomembranes.46 Analysis of these effects along with nanoflag dimensions impact on its optical properties will be described elsewhere. In summary, we have presented a novel method for controlled in situ catalyst unpinning during VLS growth of InP nanowires. The mechanism responsible for this phenomenon is asymmetrical dissolution of the nanowire tip into the catalyst, followed by Au particle unpinning, and finally epitaxial growth mediated migration along a nanowire facet. This technique enables synthesis of 2D nanomembranes with crystal structure determined by a 1D nanowire sidewall template. The insertion of stacking faults at predefined location along the nanowire determined the location of the nanoflag. The nanoflags exhibited room temperature, linearly polarized PL perpendicular to the elongation direction of the nanoflag. The nanoflag tip geometry enables significant electric field intensity enhancement applicable to nonlinear effects and as a nanoantenna. The primary nanowire and the nanomembrane can be synthesized from different materials, opening the way for the study of a variety of new 2D heterostructures and their applications. Methods. Growth. As shown in Figure 1, panel a, primary nanowires were grown by the selective-area VLS method20 on

PL from a single nanoflag, as shown in Figure 6, panel a, where a camera was used to detect a nanoflag emission after filtering laser and substrate contributions. Figure 6, panel b shows representative polarization resolved PL intensity spectrum with a distinct broad peak at 850−870 nm corresponding to InP WZ crystal structure. 42 Another weaker peak at 925 nm corresponding to InP ZB structure is related to substrate emission, as verified by PL measurement of a bare substrate. Strong polarization dependence of the flag emission, along with a nonpolarized emission from the substrate, is evident. We quantify the degree of linear polarization (DOP) as DOP = (Imax − Imin)/(Imax + Imin), where Imax and Imin stand for the PL intensity polarized perpendicular and parallel to the nanoflag elongation axis, respectively. The DOP values of the flag and substrate related emission were found to be 74% and 23%, respectively, as shown in Figure 6, panel c. The substrate PL should be unpolarized, and its nonvanishing DOP must be due to the overlap with the polarized flag PL spectrum. Figures 6, panels d and e show normalized polarization-resolved PL measurements of nanoflags oriented in two other directions rotated by 60°. The polarization axis of the emission is perpendicular to the elongation axis of the nanoflags as shown in Figure 6, panels c−e. The polarization axis and DOP values are determined by competing contributions of the optical transition selection rules in the WZ crystal structure and by the dielectric mismatch between the nanowire and surrounding air as is the case for WZ nanowires.42,43 The DOP is also influenced by contribution from differently oriented neighboring nanoflags and measurement limitations. Analysis of 15 polarization resolved PL measurements of different nanoflags resulted in DOP to be in the range of 70%−90% as explained in the Supporting Section SD. Electromagnetic simulations were carried out to investigate electric field localization and enhancement by the nanoflag tip. Finite difference time domain (FDTD) calculations were performed using the Lumerical software. The investigated 1 F


Letter

Nano Letters Notes

(111)B InP P-type substrates using a compact MOMBE system.47 The wafers were first coated by a 15 nm thick SiNx layer deposited by plasma-enhanced chemical vapor deposition (PECVD, Plasma-Therm 790) and then spin-coated by a positive PMMA resist. The samples were electron beam patterned, developed, and wet etched in buffered HF to produce circular openings in the nitride layer with diameters of 80 nm. Catalyst particles were produced by evaporation of a 4− 8 nm gold layer, and the PMMA served as a lift-off mask. InP nanowires were grown for 25 min using TMI and precracked PH3 as group III and group V precursors, respectively. The TMI flow rate corresponded to a planar growth rate of 110 nm h−1 on an InP(001) substrate at 500 °C and PH3 flow rate was 3 sccm. To displace the catalyst from the nanowire top to the side facets, the TMI flow was paused, and the temperature was increased to 515 °C at phosphine flow of 1 sccm for 5 min, see Figure 1, panel b. Following the catalyst manipulation, the nanoflag structures were grown for 40 min at 515 °C at the same TMI and PH3 flow rates as for the primary nanowire growth. PL Measurements. The optical setup is shown in Supporting Figure S7. PL measurements at room temperature were performed using Fianium supercontinum pulsed laser at 532 nm, with an acusto-optic filter. Laser output polarization was vertical and was rotated by a half waveplate. Nanoflags were measured as grown in the vertical geometry. The setup elements included a chopper connected to lock-in amplifier, and a dichroic miror with 593 nm cutoff. The laser was focused to ∼4 μm spot using 100× objective with an NA of 0.7 leading to ∼1 mW power at the sample. The PL from the sample was collected by the same objective used for focusing the laser light, dispersed by an Acton SP300i monochromator, and measured by a femto-watt detector and a lock-in amplifier. Short and long pass filters at the excitation and emission channels corspondingly were insetrted to filter the channels from unwanted light. Flip mirror with camera helped align the sample and identify PL emission. The polarization was controlled by a rotating halfwave plate and a polarizer at the monochromator input, as described in Supporting Figure S7.

■

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by Russell Berrie Nanotechnology Institute (RBNI) and the Israeli Nanotechnology Focal Technology Area on “Nanophotonics for Detection” (Grant No. 4369512). The fabrication was performed at the MicroNano Fabrication Unit (MNFU), Technion.

■

(1) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415 (6872), 617−620. (2) Algra, R. E.; Verheijen, M. a; Borgström, M. T.; Feiner, L.-F.; Immink, G.; van Enckevort, W. J. P.; Vlieg, E.; Bakkers, E. P. a M. Nature 2008, 456 (7220), 369−372. (3) Gorji Ghalamestani, S.; Ek, M.; Ganjipour, B.; Thelander, C.; Johansson, J.; Caroff, P.; Dick, K. a. Nano Lett. 2012, 12 (111), 4914− 4919. (4) Thelander, C.; Agarwal, P.; Brongersma, S.; Eymery, J.; Feiner, L. F.; Forchel, A.; Scheffler, M.; Riess, W.; Ohlsson, B. J.; Gösele, U.; Samuelson, L. Mater. Today 2006, 9 (10), 28−35. (5) Tomioka, K.; Tanaka, T.; Hara, S.; Hiruma, K.; Fukui, T. IEEE J. Sel. Top. Quantum Electron. 2011, 17 (4), 1112−1129. (6) Nakayama, Y.; Pauzauskie, P. J.; Radenovic, A.; Onorato, R. M.; Saykally, R. J.; Liphardt, J.; Yang, P. Nature 2007, 447 (June), 1098− 1101. (7) Mourik, V.; Zuo, K.; Frolov, S. M.; Plissard, S. R.; Bakkers, E. P. a. M.; Kouwenhoven, L. P. Science 2012, 336 (6084), 1003−1007. (8) Krogstrup, P.; Jørgensen, H. I.; Heiss, M.; Demichel, O.; Holm, J. V.; Aagesen, M.; Nygard, J.; Fontcuberta i Morral, A. Nat. Photonics 2013, 7 (April), 306−310. (9) Mayer, B.; Rudolph, D.; Schnell, J.; Morkötter, S.; Winnerl, J.; Treu, J.; Müller, K.; Bracher, G.; Abstreiter, G.; Koblmüller, G.; Finley, J. J. Nat. Commun. 2013, 4, 1−7. (10) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4 (1964), 89. (11) Persson, A. I.; Larsson, M. W.; Stenström, S.; Ohlsson, B. J.; Samuelson, L.; Wallenberg, L. R. Nat. Mater. 2004, 3 (10), 677−681. (12) Caroff, P.; Bolinsson, J.; Johansson, J. IEEE J. Sel. Top. Quantum Electron. 2011, 17 (4), 829−846. (13) Joyce, H. J.; Wong-Leung, J.; Gao, Q.; Tan, H. H.; Jagadish, C. Nano Lett. 2010, 10 (3), 908−915. (14) Paiman, S.; Gao, Q.; Joyce, H. J.; Kim, Y.; Tan, H. H.; Jagadish, C.; Zhang, X.; Guo, Y.; Zou, J. J. Phys. D: Appl. Phys. 2010, 43 (44), 445402. (15) Rogers, J. a.; Lagally, M. G.; Nuzzo, R. G. Nature 2011, 477 (7362), 45−53. (16) Chi, C. Y.; Chang, C. C.; Hu, S.; Yeh, T. W.; Cronin, S. B.; Dapkus, P. D. Nano Lett. 2013, 13, 2506−2515. (17) Conesa-Boj, S.; Russo-Averchi, E.; Dalmau-Mallorqui, A.; Trevino, J.; Pecora, E. F.; Forestiere, C.; Handin, A.; Ek, M.; Zweifel, L.; Wallenberg, L. R.; Rüffer, D.; Heiss, M.; Troadec, D.; Dal Negro, L.; Caroff, P.; Fontcuberta I Morral, A. ACS Nano 2012, 6 (12), 10982−10991. (18) de la Mata, M.; Leturcq, R.; Plissard, S. R.; Rolland, C.; Magen, C.; Arbiol, J.; Caroff, P. Nano Lett. 2016, 16 (2), 825−833. (19) Aagesen, M.; Johnson, E.; Sørensen, C. B.; Mariager, S. O.; Feidenhans’l, R.; Spiecker, E.; Nygård, J.; Lindelof, P. E. Nat. Nanotechnol. 2007, 2 (100), 761−764. (20) Dalacu, D.; Kam, A.; Guy Austing, D.; Wu, X.; Lapointe, J.; Aers, G. C.; Poole, P. J. Nanotechnology 2009, 20 (39), 395602. (21) Madras, P.; Dailey, E.; Drucker, J. Nano Lett. 2009, 9, 3826− 3830. (22) Oh, S. H.; Chisholm, M. F.; Kauffmann, Y.; Kaplan, W. D.; Luo, W.; Rühle, M.; Scheu, C. Science 2010, 330 (6003), 489−493. (23) Wen, C.-Y.; Tersoff, J.; Hillerich, K.; Reuter, M. C.; Park, J. H.; Kodambaka, S.; Stach, E. a.; Ross, F. M. Phys. Rev. Lett. 2011, 107 (2), 025503.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b00648. HR-TEM and SEM images of nanowires after unpinning procedure; additional SEM images of nanoflags and nanostructure arrays and structural phase transition in the middle positioned nanoflag; PL measurement setup, scattering cross-sections simulation, PL dependence on incident wave polarization, degree of polarization evaluation and normalization (PDF)

■

REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. G


Letter

Nano Letters (24) Gamalski, a. D.; Ducati, C.; Hofmann, S. J. Phys. Chem. C 2011, 115, 4413−4417. (25) Kelrich, A.; Dubrovskii, V. G.; Calahorra, Y.; Cohen, S.; Ritter, D. Nanotechnology 2015, 26 (8), 085303. (26) Schwarz, K. W.; Tersoff, J. Phys. Rev. Lett. 2009, 102 (20), 1−4. (27) Roper, S. M.; Anderson, a. M.; Davis, S. H.; Voorhees, P. W. J. Appl. Phys. 2010, 107 (2010), 114320. (28) Frolov, T.; Carter, W. C.; Asta, M. Nano Lett. 2014, 14, 3577− 3581. (29) Loitsch, B.; Rudolph, D.; Morkötter, S.; Döblinger, M.; Grimaldi, G.; Hanschke, L.; Matich, S.; Parzinger, E.; Wurstbauer, U.; Abstreiter, G.; Finley, J. J.; Koblmüller, G. Adv. Mater. 2015, 27, 2195−2202. (30) Plissard, S. R.; van Weperen, I.; Car, D.; Verheijen, M. a.; Immink, G. W. G.; Kammhuber, J.; Cornelissen, L. J.; Szombati, D. B.; Geresdi, A.; Frolov, S. M.; Kouwenhoven, L. P.; Bakkers, E. P. a. M. Nat. Nanotechnol. 2013, 8 (11), 859−864. (31) Zi, Y.; Jung, K.; Zakharov, D.; Yang, C. Nano Lett. 2013, 13, 2786−2791. (32) Fonseka, H. A.; Caroff, P.; Wong-Leung, J.; Ameruddin, A. S.; Tan, H. H.; Jagadish, C. ACS Nano 2014, 8 (7), 6945−6954. (33) Dick, K. a.; Kodambaka, S.; Reuter, M. C.; Deppert, K.; Samuelson, L.; Seifert, W.; Wallenberg, L. R.; Ross, F. M. Nano Lett. 2007, 7, 1817−1822. (34) Quitoriano, N. J.; Wu, W.; Kamins, T. I. Nanotechnology 2009, 20, 145303. (35) Dalacu, D.; Kam, A.; Austing, D. G.; Poole, P. J. Nano Lett. 2013, 13, 2676−2681. (36) Oliveira, D. S.; Tizei, L. H. G.; Li, A.; Vasconcelos, T. L.; Senna, C. A.; Archanjo, B. S.; Ugarte, D.; Cotta, M. A. Nanoscale 2015, 7 (29), 12722−12727. (37) Dayeh, S. A.; Wang, J.; Li, N.; Huang, J. Y.; Gin, A. V.; Picraux, S. T. Nano Lett. 2011, 11 (10), 4200−4206. (38) Sibirev, N. V.; Timofeeva, M. a.; Bol’shakov, a. D.; Nazarenko, M. V.; Dubrovskiĭ, V. G. Phys. Solid State 2010, 52 (7), 1531−1538. (39) Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Nature 2001, 409 (6816), 66−69. (40) Martín-Palma, R. J.; Manso, M.; Torres-Costa, V. Sensors 2009, 9 (7), 5149−5172. (41) Boztug, C.; Sánchez-Pérez, J. R.; Sudradjat, F. F.; Jacobson, R.; Paskiewicz, D. M.; Lagally, M. G.; Paiella, R. Small 2013, 9 (4), 622− 630. (42) Mishra, a.; Titova, L. V.; Hoang, T. B.; Jackson, H. E.; Smith, L. M.; Yarrison-Rice, J. M.; Kim, Y.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C. Appl. Phys. Lett. 2007, 91 (26), 263104. (43) Ba Hoang, T.; Moses, A. F.; Ahtapodov, L.; Zhou, H.; Dheeraj, D. L.; Van Helvoort, A. T. J.; Fimland, B. O.; Weman, H. Nano Lett. 2010, 10 (8), 2927−2933. (44) Foster, A. P.; Bradley, J. P.; Gardner, K.; Krysa, A. B.; Royall, B.; Skolnick, M. S.; Wilson, L. R. Nano Lett. 2015, 15 (3), 1559−1563. (45) Kühn, S.; Håkanson, U.; Rogobete, L.; Sandoghdar, V. Phys. Rev. Lett. 2006, 97 (1), 1−4. (46) Pecora, E. F.; Walsh, G. F.; Forestiere, C.; Handin, A.; RussoAverchi, E.; Dalmau-Mallorqui, A.; Canales-Mundet, I.; Fontcuberta i Morral, A.; Negro, L. D. Nanoscale 2013, 5, 10163. (47) Hamm, R. A. J. Vac. Sci. Technol., A 1994, 12 (5), 2790.

H


InP Nanoflag Growth from a Nanowire Template by in Situ Catalyst

Recommend Documents