Crystalline Niobium Carbide Superconducting Nanowires Prepared by

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Crystalline Niobium Carbide Superconducting Nanowires Prepared by Focused Ion Beam Direct Writing Fabrizio Porrati,*,† Sven Barth,‡ Roland Sachser,† Oleksandr V. Dobrovolskiy,† Anja Seybert,§ Achilleas S. Frangakis,§ and Michael Huth† Downloaded via RUTGERS UNIV on August 8, 2019 at 08:07:09 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Physikalisches Institut, Goethe-Universität, Max-von-Laue-Strasse 1, D-60438 Frankfurt am Main, Germany Institute of Materials Chemistry, TU Wien, Getreidemarkt 9/BC/02, A-1060 Wien, Austria § Buchmann Institute for Molecular Life Sciences, Goethe-Universität, Max-von-Laue-Strasse 15, D-60438 Frankfurt am Main, Germany ‡

ABSTRACT: Superconducting planar nanostructures are widely used in applications, e.g., for highly sensitive magnetometers and in basic research, e.g., to study finite size effects or vortex dynamics. In contrast, 3D superconducting nanostructures, despite their potential in quantum information processing and nanoelectronics, have been addressed only in a few pioneering experiments. This is due to the complexity of fabricating 3D nanostructures by conventional techniques such as electron-beam lithography and to the scarce number of superconducting materials available for direct-writing techniques, which enable the growth of 3D free-standing nanostructures. Here, we present a comparative study of planar nanowires and free-standing 3D nanowires fabricated by focused electron- and ion (Ga+)-beam induced deposition (FEBID and FIBID) using the precursor Nb(NMe2)3(N-t-Bu). FEBID nanowires contain about 67 atomic percent C, 22 atomic percent N, and 11 atomic percent Nb, while FIBID samples are composed of 43 atomic percent C, 13 atomic percent N, 15.5 atomic percent Ga, and 28.5 atomic percent Nb. Transmission electron microscopy shows that FEBID samples are amorphous, while FIBID samples exhibit a fcc NbC polycrystalline structure, with grains about 15−20 nm in diameter. Electrical transport measurements show that FEBID nanowires are highly resistive following a variable-range-hopping behavior. In contradistinction, FIBID planar nanowires become superconducting at Tc ≈ 5 K. In addition, the critical temperature of free-standing 3D nanowires is as high as Tc ≈ 11 K, which is close to the value of bulk NbC. In conclusion, FIBID-NbC is a promising material for the fabrication of superconducting nanowire single-photon detectors (SNSPD) and for the development of 3D superconductivity with applications in quantum information processing and nanoelectronics. KEYWORDS: niobium carbide, superconducting nanowires, direct writing, 3D nanoprinting, focused electron and ion beam induced deposition

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high-sensitivity magnetometry based on superconducting quantum interference devices5 and, among others, studies on vortex dynamics in planar nanostructures,6,7 nanoarchitectures,8 and nanopatterned superconductors,9 have become a focus of intense research activity. On the fabrication side, several techniques are available to define the geometry and tune the properties of superconductors. Pseudo-ordered nanostructures were prepared by self-assembling colloidal nanoparticles10 or by using nano-

uperconducting nanostructures exhibit a number of intriguing phenomena, e.g., vortex ratchet1 and quantum size effects,2 that are absent in bulk superconductors. Shrinking the size of the sample close to its characteristic lengths, i.e., penetration depth (λL) and coherence length (ξ0), induces, for example, a decrease of critical temperature (Tc) in thin films, superconductor-to-insulator transitions in thin films and nanowires, and the generation of phase-slip centers caused by thermal and quantum fluctuations in one-dimensional systems. Superconductivity at the nanoscale enables applications in the fields of quantum information, with superconducting circuits based on Josephson junctions3 or with nanowires working as single-photon detectors.4 Moreover © 2019 American Chemical Society

Received: January 3, 2019 Accepted: May 2, 2019 Published: May 2, 2019 6287

DOI: 10.1021/acsnano.9b00059 ACS Nano 2019, 13, 6287−6296

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Cite This: ACS Nano 2019, 13, 6287−6296

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ACS Nano templates, such as anodic aluminum oxide membranes, filled to produce nanowires.11 Furthermore, extended planar structures, e.g., superconducting networks, were obtained by the deposition through shadow masks,12 a process that is limited in resolution by the mask thickness. However, nanolithographic techniques allow the fabrication of large-scale ordered nanostructures with resolution below 10 nm. In this field, most popular are nanoimprint lithography13 and electron-beam lithography,14 which were employed, e.g., to fabricate nanoSQUIDS,5,15 superconducting logic devices,16 nanowires,17−19 and planar nanostructures.7 If large-scale nanostructuring is not required, direct-writing techniques such as scanning probe lithography20 and focused electron- and ion-beam induced processing21−23 come into play. These mask-less techniques notably simplify the fabrication procedure, which is especially advantageous for the fabrication of complex 3D nanostructures.24,25 Among the direct-writing techniques, focused electron- and ion-beam induced deposition (FEBID and FIBID, respectively) is often chosen for nanofabrication in research and prototype applications. In FEBID and FIBID, the electron and ion beam of a scanning electron microscope (SEM) or focused ion beam (FIB) microscope dissociates the adsorbed molecules of a precursor gas injected in the microscope chamber, forming the sample during the rastering process.22,23 In the past decade, thanks to the advances in precursor synthesis, deposition processing, and purification techniques, a number of magnetic materials, superconductors, alloys, intermetallic compounds, multilayers, and metamaterials has become available to the scientific community.26−28 In the field of superconductivity, the most employed material is an amorphous W-based superconductor grown by FIBID using Ga+ as ion source and W(CO)6 as precursor gas, with critical temperature Tc of ∼5 K.29 The properties of this superconductor have been extensively investigated.30−32 This material exhibits a Bardeen−Cooper−Schrieffer (BCS)-like behavior with very homogeneous superconducting properties, as shown by tunneling spectroscopy experiments,33 allowing studies in the field of vortex dynamics,6,34,35 measurements of Andreev reflection in ferromagnet−superconductor nanocontacts36,37 and of proximity-effects in metal-superconducting nanowires38−41 and investigations in superconductor-molecule-superconductor junctions.42 Other superconductor materials, i.e., C−Ga−O and Mo−Ga−C−O nanocomposites, with Tc values of 7 and 3.8 K, respectively, were prepared by FIBID with the precursors phenanthrene (C14H10) and Mo(CO)6.43,44 Moreover, gallium-free superconducting nanostructures were recently fabricated by helium ion microscopy (HIM) and FEBID by using W(CO)6,45,46 Mo(CO)6 in the presence of water,47 and (CH3CH2)4Pb,48 with Tc values between 2 and 7.2 K. One notes that the majority of these materials are amorphous superconductors with a high degree of disorder, which limits their application because statistical fluctuations of the order parameter can lead to spatial variations of Tc.49,28 Focused ion- and electron-beam induced deposition technologies allow the fabrication of nanostructures of any dimension with a resolution of a few nanometers. The majority of investigations up-to-date involves the fabrication of planar nanostructures. However, two decades of 3D free-standing nano-objects have already been successfully implemented.50−55 Recently, a step forward toward the fabrication of complex 3D nanostructures has been taken with the introduction of

pattern-generation approaches,56−58 which were used to grow complex nanoarchitectures for plasmonics and magnetism.59,60 In the field of superconductivity, free-standing 3D nanostructures have been addressed only in a few pioneering works.46,61−63 Vertical W-based nanowires and air-bridge nanostructures were grown by Ga+-FIBID using the W(CO)6 precursor. These structures were proven to be superconducting with Tc ≈ 5 K.61,62 Furthermore, three-dimensional W-based pick-up loops were coupled to a superconducting quantum interference device to detect both the out-of-plane and the inplane magnetic field components of nanomagnets.63 Very recently, Cordoba et al. have fabricated high-resolution crystalline fcc WC1−x vertical hollow nanowires by He+FIBID microscopy, with a superconducting critical temperature of 6.4 K.46 In this work, we investigate planar and freestanding 3D nanowires grown by focused electron- and ion-beam induced deposition using the precursor Nb(NMe2)3(N-t-Bu). The nanowires are characterized in composition by energydispersive X-ray spectroscopy (EDX) and in microstructure by transmission electron microscopy (TEM). Electrical transport measurements are carried out in the range 2−285 K. At low temperatures, for T ≤ 20 K, the superconducting transition of the FIBID nanowires is investigated by magneticfield- and temperature-dependent transport measurements.

RESULTS AND DISCUSSION Fabrication. The electron beam parameters used for the growth of FEBID planar nanowires were 5 kV for the acceleration voltage, 1.6 nA for the beam current, 20 nm for the pitch, and 1 μs for the dwell time, respectively. The ionbeam parameters used during the fabrication of FIBID planar nanowires were 30 kV for the acceleration voltage, 10 pA for the beam current, 30 nm for the pitch and 200 ns for the dwell time, respectively. The nanowires had a length of about 10 μm and a width of about 500 nm; see Figure 1a. The thickness was about 80 and 100 nm, for FEBID nanowires written at room temperature and 250 °C, respectively, and 40 nm for FIBID nanowires, as measured by atomic force microscopy (AFM) in non-contact mode (nanosurf, easyscan2). The samples were fabricated on Si (p-doped)/SiO2 (200 nm) substrates. The 50 nm thick Au/Cr contacts for electrical transport measurements were prepared by standard UV lithography. Free-standing 3D nanowires were written directly onto the Au electrodes by FIBID (see Figure 1b) with 30 kV acceleration voltage, 1 pA beam current, and 1 ms dwell time. Deposit Composition. EDX measurements were carried out in situ on samples of 2 μm × 2 μm lateral size and thickness of about 100 nm by using an electron-beam energy of 5 keV. The EDX analysis of FEBID samples fabricated with 5 kV acceleration voltage and 1.6 nA electron current reveals an elemental composition of about 67 atomic percent C, 22 atomic percent N, and 11 atomic percent Nb. The composition of samples prepared with 5 keV acceleration voltage was found to be independent of the beam current in the range of 0.4−6.3 nA and dwell time for 1 and 100 μs. A similar result was found using 10 kV acceleration voltage for the deposition, keeping constant the other electron beam parameter. To tune the composition of the deposits, a series of samples was fabricated with 5 kV acceleration voltage and 1.6 nA electron beam current by varying the substrate temperature from room temperature up to 300 °C. By increasing the substrate temperature, the elemental composition changed as 6288


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Figure 2. EDX compositional analysis result of FEBID planar nanowires grown at different substrate temperatures. The contents of Nb and N increase in the temperature range considered, while the C content decreases. At about 305 °C, the Nb(NMe2)3(N-t-Bu) precursor decomposes thermally.

depicted in Figure 3c. The blue line corresponds to the sample fabricated by FIBID and irradiated by an electron beam at 5 keV after growth. The peaks match well with those expected for fcc NbC with a lattice constant of a = 4.47 Å.64 The red line refers to the as-grown sample fabricated by FIBID. The peak positions correspond to a fcc NbC lattice constant of a = 4.53 Å. It is known that electron irradiation curing improves crystallinity in FEBID materials.31,65 Similarly, in the present case, the irradiated FIBID sample has a smaller lattice constant than the as-grown sample, indicating a higher crystal quality. Figure 3d is a zoomed-in view of of Figure 3b. Here, interference fringes belonging to the (111) and (200) atomic planes, respectively, with a planar distance of 2.58 and 2.23 Å, are visible. Free-standing 3D nanowires for TEM investigations were grown on a standard Cu lift-out grid. In Figure 4a are depicted a HRTEM micrograph and the corresponding SAED pattern for a 3D nanowire. It is found that 3D nanowires and planar nanowires have the same NbC polycrystalline structure. The radial intensity of the diffraction pattern of the 3D nanowire is shown in Figure 4b; see the brown line. The peaks relate to a lattice constant of a = 4.47 Å. Magnetotransport Properties. Transport measurements were carried out on FEBID planar nanowires prepared either at room temperature or at 250 °C. A pair of samples were measured in the as-grown state. We label these samples as s1, prepared at RT, and s3, grown at 250 °C. A pair of other samples were measured after a low-energy electron irradiation treatment carried out at 5 keV acceleration voltage and 1.6 nA beam current. We denote these samples as s2, prepared at room temperature and irradiated with 982 nC/μm2 dose, and s4, fabricated at 250 °C and irradiated with a 211 nC/μm2 dose. The electrical conductivity of the irradiated samples increased monotonically during treatment. In the case of s4, the irradiation was interrupted after reaching apparent saturation. The room temperature electrical resistivity of the four samples were ρs1 = 8.5 × 105 μΩ cm, ρs2 = 1.6 × 105 μΩ cm, ρs3 = 1.0 × 104 μΩ cm, and ρs4 = 6.4 × 103 μΩ cm for s1− s4, respectively. In Figure 5, we report the temperaturedependent electrical conductivity measured for the four samples. All the samples show insulating behavior at low temperature. In panels a and b of Figure 6, the logarithm of the normalized conductivity is plotted versus 1/T1/4 and 1/T1/2,

Figure 1. (a) SEM micrograph of a NbC planar nanowires prepared by FIBID contacted with Au electrodes for transport measurements. The nanowire has a width of about 500 nm, a length of 10 μm, and a thickness of 80 nm. (b) SEM micrograph of a freestanding 3D NbC FIBID nanowire of width 170 nm. The distance between the voltage probes (V+, V−) is about 1.5 μm.

67−50.2 atomic percent for C, 22−24.4 atomic percent for N, and 11−25.4 atomic percent for Nb, respectively; see Figure 2. Above 300 °C, thermal decomposition was observed. The corresponding thin film showed a composition of about 35.5 atomic percent C, 30.0 atomic percent N, and 34.5 atomic percent Nb. The typical elemental composition of the FIBID deposits was about 42.9 atomic percent C, 12.9 atomic percent N, 15.5 atomic percent Ga, and 28.7 atomic percent Nb. Microstructural Characterization. The microstructure of the samples was investigated by high-resolution TEM (HRTEM). In Figure 3a, a TEM micrograph of a NbC sample fabricated by FEBID is shown. In the inset, the corresponding selected area electron diffraction (SAED) pattern is depicted. The diffraction pattern does not show any well-defined ring, as is characteristic for amorphous materials. In Figure 3b, a HRTEM micrograph and the corresponding SAED pattern for a sample grown by FIBID are shown. The material exhibits a polycrystalline structure with partially touching nanocrystallites of about 15−20 nm diameter. The radial intensity of the diffraction pattern is 6289


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Figure 3. (a) TEM micrograph of a planar nanowire grown by FEBID. The inset depicts the corresponding SAED pattern. In the upper-left half of the figure is visible the Pt-based FEBID composite used to protect the sample during the lamella preparation. (b) HRTEM micrograph of a planar nanowire grown by FIBID. The corresponding SAED pattern is shown in the inset. (c) Radial intensity of the diffraction pattern. Lines refer to the as-grown nanowire relating to the inset in panel b, in red, and to a nanowire treated with electron irradiation after growth, in blue. The expected peaks of fcc NbC are shown in green. (d) Zoomed-in area of panel b showing (111) and (200) lattice planes.

Figure 4. (a) HRTEM micrograph and SAED pattern (inset) of a free-standing 3D NbC FIBID nanowire. (b) Radial intensity of the diffraction pattern. In the graph are compared the peaks obtained for the as-grown planar nanowire, red line, the 3D nanowire, brown line, and the expected peaks for fcc NbC.

respectively, as this is the expected behavior for Mott-type variable-range-hopping (VRH)66 and correlated VRH, respectively.67 For all of the samples, the best correspondence is found if Mott-VRH is assumed. This is particularly clear for sample s1.

FIBID planar nanowires were prepared at room temperature with the above-mentioned ion beam parameters. Electrical transport measurements were carried out either in the asgrown state or after electron irradiation treatment performed at 5 keV with a 22 nCμ/m2 dose. The room-temperature specific 6290


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temperature curve is negative. At low temperatures, the resistivity drops and superconductivity sets it. The critical temperature, defined at the half of the resistivity in the transition region, is Tc = 5.0 K for the as-grown sample and Tc = 5.4 K for the irradiated sample. In Figure 8a, the behavior of

Figure 5. Electrical conductivity normalized at 285 K vs temperature. Samples s1 and s2 were grown at room temperature and measured in the as-grown state and after electron irradiation, respectively. Samples s3 and s4 were grown at 250 °C and investigated as-grown and after irradiation, respectively.

Figure 8. (a) Temperature-dependent electrical resistivity of a planar NbC FIBID nanowires measured in the as-grown state as a function of the external magnetic fields from 0 T up to 8 T. (b) Electrical resistivity vs external magnetic field for various temperatures from 2 up to 5 K. The curves are extracted from the data shown in panel a. At 2 K and 1 T, the sample is no longer completely superconducting. (c) Temperature dependence of the upper critical field for a planar nanowire in the as-grown state (red) and in the irradiated state (blue). Fits are made using the Ginzburg−Landau (GL) relation Bc2(T) = Bc2(0)(1 − (T/Tc)2).

the resistivity of the as-grown sample is plotted at low temperatures as a function of an external magnetic field applied perpendicular to the sample surface. The values of Tc, both in zero and in applied magnetic field, are in the same range of the values found in W-based FIBID nanowires.29,62 Furthermore, in Figure 8b is plotted the resistivity versus magnetic field for various temperatures in the range of 2−5 K. From the plot, we deduce that the critical field necessary to destroy superconductivity is about 1 T at 2 K, which is similar to the value obtained in W-based FIBID nanowires by Sadki et al.29 but smaller than the corresponding value of about 2.5 T measured by Kasumov et al.42 In Figure 8c is reported the temperature dependence of the upper critical field Bc2, defined at the 90% value of the resistivity in the transition region for the two samples investigated. By extrapolating the value of the upper critical field to zero temperature and by using the relation Bc2(0) = Φ/2π0ξ(0)2, the value of the coherence length ξ(0) can be deduced. In particular, we find ξ(0) equals 5.94 and 5.79 nm for the sample measured in the as-grown state and for the sample treated with electron irradiation, respectively. The penetration depth λ at zero temperature can be estimated from the expression68 λ(0) [nm] = 1.05 × 102 ρn [μΩ cm] /Tc [K] , which gives λ(0) = 1101 nm and λ(0) = 1030 nm for as-grown and irradiated nanowire, respectively. These values are about 20% higher than those found for W-based FIBID nanowires.6,46 Finally, the Ginzburg−Landau parameter is κ = λ/ξ = 185. Free-standing 3D nanowires were measured in the as-grown state. The room-temperature specific resistivity was about ρRT = 380 μΩ cm. The resistivity slightly decreased by lowering the temperature, with a resistivity ratio at ρRT/ρ20K = 1.02. In Figure 9, we show the low-temperature resistivity behavior of one of the measured nanowires. Superconductivity sets in at Tc

Figure 6. Logarithmic electrical conductivity vs (a) 1/T1/4 and (b) 1/T1/2. Linear fits indicate that the samples follow Mott-variable range-hopping.

resistivity was about 550 μΩ cm for the as-grown sample and 520 μΩ cm for the irradiated sample. In Figure 7, the temperature-dependent resistivity is reported in the range of 2−290 K. For both samples, the slope of the resistivity versus

Figure 7. Temperature-dependent electrical resistivity of planar NbC FIBID nanowires measured in the as-grown and postirradiated states, respectively. (a) The resistivity of the nanowires slowly increases by lowering the temperature. At about 5 K, superconductivity sets in. (b) At low temperatures, a shift in Tc from 5 to 5.4 K is measured for the irradiated sample. 6291


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Figure 9. Temperature-dependent electrical resistance of a freestanding 3D NbC FIBID nanowire. In the as-grown state, the critical temperature is Tc ≈ 8.1K, shown with a green line. Inset: derivative of the resistance in the as-grown state. After the application of 1 μA of DC current for about 80 min, the nanowire exhibits a change in the temperature dependence of the resistance, with lower resistance and Tc ≈ 11.4 K.

= 8.1 K; see the green line in Figure 9. The resistivity already starts to decrease at about 12 K, as shown in the inset by the derivative of the resistivity, which is the critical temperature of bulk NbC. This is likely indicative of the starting formation of a Josephson network. The transition into the superconducting state at low temperatures is not complete as shown by the finite resistance. The measurement was carried out with a constant current of 1 μA. By applying such a current for 80 min, a transition into a lower resistivity state takes place; see the blue line in Figure 9. This transition was observed in several 3D nanowires but not in planar nanowires. As a consequence of this effect, the resistance of the sample in the normal state dropped to ρRT = 253 μΩ cm and Tc shifted to 11.4 K, which is close to the value of bulk NbC. We speculatively attribute this transition to a graphitization of the matrix as a consequence of the electromigration effect.69,70 Moreover, Joule heating effects may also play a role in the graphitization process, as is known for FEBID materials,71 especially considering freestanding objects, where rapid thermalization is strongly hindered as compared with planar structures. The graphitization of the matrix improves the conductivity of the wire,72,71 as shown in the normal state, and the link between superconducting NbC grains, shifting Tc to higher temperatures. In Figure 10a, we depict the resistivity of the nanowire at low temperatures as a function of the external magnetic field, which is applied perpendicular to the sample surface and has an angle of about 22° with respect to the main axis of the nanowire. The analysis of the upper critical field leads to a coherence length of ξ(0) = 5.85 nm. The penetration depth is λ(0) = 471 nm, which is less than half the value obtained for planar nanowires. Finally, the ratio κ = λ/ξ ≈ 84. Here we note that, because the critical field Hc ≈ Δ ≈ Tc,73 the lower critical field H3D c1 of the 3D nanowires is four times larger than the lower critical field H2D c1 of the planar nanowires, 2D 3D 2D i.e., H3D c1 ≈ 4Hc1 , where we have used Tc ≈ 2Tc and the equation Hc ≈ Hc1Hc2 . Finally, in Figure 10b, we plot the current−voltage characteristic of the 3d nanowire as a function of temperature. Typical critical current densities of Jc ≈ 0.1 MA/cm2 are obtained. From the literature, it is known that the critical temperature of NbC depends on the C-to-Nb ratio.74 In particular, Tc ≈ 12

Figure 10. (a1) Temperature-dependent electrical resistance of a freestanding 3D NbC FIBID nanowire as a function of external magnetic field between 0 and 8 T. The nanowire is measured after application of 1 μA DC current; see Figure 9. (a2) Temperature dependence of the upper critical field Bc2 for the 3D nanowire. The fit is done using the GL relation. (b1) VI characteristic for various temperatures. After repeated measurements with a maximum current of 20 μA, the nanowire broke. (b2) Critical current density Js vs temperature. The red line is a guide for the eye.

K is obtained for [C]/[Nb] ≈ 1. This value drops with decreasing carbon content, reaching Tc ≈ 1 K for [C]/[Nb] ≈ 0.8. Accordingly, because the planar nanowires of the present work have Tc ≈ 5 K, a ratio of [C]/[Nb] ≈ 0.9 may be deduced, while for the 3D nanowires, the ratio is [C]/[Nb] ≈ 1. The reason for the difference between planar and 3D nanowires has to be attributed to the growth process, more specifically to the use of a different ion current, which was, respectively, 10 and 1 pA. We note that the value of Tc ≈ 11 K obtained for NbC 3D nanowires is about twice as large as the value obtained for the W-based superconductor, currently the most used material for fabricating nanostructures by direct writing.6,29,32−39,41,46,61−63 The origin of the superconductivity in the W-based superconductor is still under debate. In particular, due to the amorphous character of the material, it is unclear which role played is by the gallium atoms in formation of the superconducting state. This issue applies to the whole family of amorphous superconductors grown by FIBID using a gallium source.29,43,44 In contrast, the origin of superconductivity in the nanowires investigated here is clearly to be attributed to the formation of NbC nanocrystallites. During the fabrication process, the gallium ions generate a cascade of secondary electrons, which are responsible for the decomposition of the precursor. The gallium atoms are present in the matrix of the material together with carbon and oxygen. The composition of the matrix does not affect directly Tc, whose value, at least in the 3D nanowires, is typical for bulk NbC. 6292


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that the color of the product as well as the boiling point differ from the reference, and changes in color can be observed when the targeted compound is exposed to atmosphere. The synthesis and handling of the precursor requires inert gas/Schlenk techniques to prevent hydrolysis. All solvents were dried over sodium wire and thoroughly degassed before use by three freeze−pump−thaw cycles. LiNMe2, tbutylamine, NbCl5, and solvents and have been purchased from Sigma-Aldrich. 1H and 13C{1H} nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AVANCE-250 spectrometer and referenced to SiMe4. The NbCl3(NtBu)(py)2 starting compound has been prepared according to literature. A total of 5 g (11.7 mmol) of [NbCl3(NtBu)(py)2] was suspended in 40 mL of hexane and cooled to −89 °C, and a suspension of 1.79 g (35 mmol) of LiNMe2 in 20 mL of hexane was added. The mixture was slowly heated to room temperature and stirred for 24 h. After filtration to separate the formed LiCl, the solvent was removed under reduced pressure. The product was distilled under reduced pressure (76 °C, 0.01 mbar) to yield a slightly yellow product that crystallized upon cooling. The following spectroscopy data were obtained. 1H NMR (C6D6, 20 °C, δ): 1.40 (9 H, s, NC(CH3)3), 3.17 (18 H, s, N(CH3)2). 13C{1H} NMR (25 °C, 100.6 MHz, C6D6, δ in ppm): 33.4 (C(CH3)2), 47.0 (C(CH3)3), 48.7 (N(CH3)2). Fabrication. To fabricate the samples, we employed a dual-beam FIB/SEM microscope (FEI, Nova NanoLab 600) equipped with a Schottky electron emitter. A standard FEI gas-injection system (GIS) was used to inject the precursor in the SEM via a capillary with inner and outer diameter of about 0.5 and 0.8 mm, respectively. The temperature of the precursor was 30 °C. The distance capillarysurface of the sample was about 100 μm. The substrate−capillary angle was about 35◦ during FEBID processing. The angle between electron beam and ion beam was 52°. The angle between ion beam and capillary was 35°. The basis pressure of the microscope chamber was 5 × 10−7 mbar. The pressure during deposition was about 1.2 × 10−6 mbar. In general, the fundamental building blocks to fabricate 3D freestanding nano-objects are segments with an inclination angle, which is a function of the precursor flux, the beam parameters and of the lateral speed of the beam.79,80 The lateral speed of the beam is the key parameter to sweep between vertical nanowires (no beam movement) and planar nanostructures (high lateral speed). Recently, the fabrication of complex 3D nanostructures became possible with the introduction of pattern-generation approaches;56−58 however, simple structures can be prepared within a heuristic approach, as has been done in the present work. The four arms of the nanostructure were written in “parallel”, meaning that the electron beam dwells 1 ms on each arm before moving to the next arm. After a 4 ms cycle, the beam was parked out of the area of interest for additional 4 ms to allow for precursor replenishment. Each cycle of 8 ms was repeated 25 times. Next, the beam was moved laterally by Δr = 0.19 nm to slope the arms. The lateral speed was 0.95 nm/s. The arms used as pick-up electrodes were deposited along the main arm of the structure. Therefore, possible structural inhomogeneities at the apex of the structure do not influence the four-probe measurement. Sample Characterization. TEM lamellae were prepared by focused ion beam cutting and lift-out with a nanomanipulator. Before milling, the samples were protected by a 200 nm Pt-based overlayer grown by FEBID. TEM investigations were carried out on a Tecnai F30 equipped with a Schottky field emitter at 300 kV. Images were recorded with a Gatan Ultrascan 4000 charge-coupled device camera using the Gatan DigitalMicrograph software. Electrical transport measurements were carried out in the temperature range of 2−285 K inside a self-made variable-temperature-insert (VTI) mounted in a 4 He cryostat equipped with a 11 T superconducting solenoid. The measurements were performed employing a Keithley Sourcemeter 2635B and an Agilent 34420A nanovoltmeter.

Finally, the composition of the matrix, and particularly the presence of gallium atoms, is expected to be decisive in the formation of sufficiently strongly Josephson-coupled superconducting regions with a Tc close to the bulk value of stoichiometric NbC. To mention one of possible applications of NbC-FIBID nanowires, we note that dirty-limit superconductors are promising materials for superconducting nanowire single photon detectors (SNSPDs).4 The operating principle of SNSPDs is based on the formation of a normal hot-spot region in a thin current-biased superconducting nanowire due to the absorption of a photon and registering of the associated voltage drop using a read-out circuit. The main advantages of SNSPDs over silicon-based ones are cryogenic operating temperatures that substantially reduces noise and the lower values of the superconducting energy gap, Δ, the minimum energy requested to a photon to create a quasiparticle. The smaller the gap Δ, the higher the sensitivity and the efficiency of the device. The promising results of the examination of NbCFIBID structures as SNSPDs, and its comparison with ones made of NbRe or NbN, the last material being most widely used for SNSPDs so far, will be reported elsewhere. Here, we mention that because putting a superconductor in contact with a ferromagnetic layer was demonstrated to produce faster relaxation processes,75 it should be feasible to further reduce the response time of NbC-FIBID detectors by capping them with a ferromagnetic CoFe-FEBID layer76 in the same SEM without breaking the vacuum. In addition, while high coupling efficiencies between the light source and the detector is still a challenge due to the required focusing of the input light on a detector, which is typically defined as a square (or a circle) with few micrometers of edge length (or diameter), the direct writing by FIBID and FEBID allows one to fabricate nanowire structures directly on the tip of the optical fiber.77

CONCLUSIONS In this work, we have characterized the composition, the microstructure, and the electrical behavior of planar nanowires and freestanding 3D nanowires fabricated by electron- and ionbeam induced deposition using the precursor Nb(NMe2)3(Nt-Bu). Transmission electron microscopy shows that FEBID planar nanowires are amorphous, while FIBID planar nanowires and freestanding 3D nanowires are constituted by fcc NbC crystallites with a size of 15−20 nm. Electrical transport measurements indicate that FEBID nanowires are insulating, following a variable-range-hopping temperature-dependence behavior. In contradistinction, FIBID nanowires are metallic or quasi-metallic in the normal state. At low temperatures, superconductivity sets in with critical temperature Tc ≈ 5 K for FIBID planar nanowires and Tc ≈ 11 K for FIBID 3D nanowires, respectively. According to the literature, this value is so far the highest value measured for 3D superconducting nanostructures. In conclusion, FIBID-NbC is a promising material for the fabrication of superconducting nanowire single photon detectors (SNSPD). Furthermore, the ability to grow truly 3D freestanding NbC nanowires support the development of 3D superconductivity with applications in quantum information processing and nanoelectronics. METHODS AND EXPERIMENTAL DETAILS

AUTHOR INFORMATION

Precursor. The synthesis of the Nb(NMe2)3(N-t-Bu) precursor was carried out in a modified procedure to the published salt elimination reaction described by Baunemann et al.78 We have to note

Corresponding Author

*E-mail: [email protected]. 6293


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Fabrizio Porrati: 0000-0003-1925-9437 Sven Barth: 0000-0003-3900-2487 Oleksandr V. Dobrovolskiy: 0000-0002-7895-8265 Michael Huth: 0000-0001-7415-465X Notes

The authors declare no competing financial interest.

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