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Charging effects and surface potential variations of Cu-based nanowires
TSF-34859; No of Pages 9 Thin Solid Films xxx (2015) xxx–xxx
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Charging effects and surface potential variations of Cu-based nanowires D. Nunes a,⁎, T.R. Calmeiro a, S. Nandy a, J.V. Pinto a, A. Pimentel a, P. Barquinha a, P.A. Carvalho b,c, J.C. Walmsley d, E. Fortunato a,⁎, R. Martins a,⁎ a
i3N/CENIMAT, Department of Materials Science, Faculty of Science and Technology, Universidade NOVA de Lisboa, Campus de Caparica, 2829-516 Caparica, Portugal SINTEF Materials and Chemistry, PB 124 Blindern, NO-0314, Oslo, Norway CeFEMA, Instituto Superior Técnico, Universidade de Lisboa, 1049-001, Lisboa, Portugal d SINTEF Materials and Chemistry, Materials and Nanotechnology, Høgskoleringen 5, 7034 Trondheim, Norway b c
a r t i c l e
i n f o
Article history: Received 15 June 2015 Received in revised form 27 November 2015 Accepted 29 November 2015 Available online xxxx Keywords: Cu-based nanowires Electrostatic force microscopy Kelvin probe force microscopy, charge mapping Dielectric polarization Capacitive effects
a b s t r a c t The present work reports charging effects and surface potential variations in pure copper, cuprous oxide and cupric oxide nanowires observed by electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM). The copper nanowires were produced by wet synthesis, oxidation into cuprous oxide nanowires was achieved through microwave irradiation and cupric oxide nanowires were obtained via furnace annealing in atmospheric conditions. Structural characterization of the nanowires was carried out by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. During the EFM experiments the electrostatic field of the positive probe charged negatively the Cu-based nanowires, which in turn polarized the SiO2 dielectric substrate. Both the probe/nanowire capacitance as well as the substrate polarization increased with the applied bias. Cu2O and CuO nanowires behaved distinctively during the EFM measurements in accordance with their band gap energies. The work functions (WF) of the Cu-based nanowires, obtained by KPFM measurements, yielded WFCuO N WFCu N WFCu2O. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Copper-based nanowires have attracted a growing interest in advanced materials and nanotechnology research [1–3] owing to the high electrical conductivity of copper [4] and the p-type semiconductor behavior of both cuprous oxide (Cu2O) [1,5–8] and cupric oxide (CuO) [9,10], which present energy band gaps of, respectively, 2.17 eV [1] and 1.4 eV [11]. These materials are interesting for a plethora of nanooptoelectronic applications [12–14], ranging from solar cells [15], field effect transistors [12], transparent flexible electrodes integrated in capacitive touch sensors [16] to glucose [10] and gas sensors [9]. The oxidation of Cu nanowires has been extensively studied over the years [1,17–19], in which the thermal route is usually adopted due to its simplicity and high-quality/low-cost relation [20,21]. Although the method is effective for the production of CuO nanowires, high fractions of Cu2O nanowires are difficult to achieve. An alternative route based on microwave irradiation [22–24] has proven to be capable of producing nearly pure Cu2O nanowires with suitable efficiency/cost balance [1]. Nevertheless, when device integration is sought, the final performance will depend critically on the nanowire characteristics and a reproducible performance demands uniform electrical properties along each nanowire as well as from wire to wire in the same batch, which remains challenging. Thus, ⁎ Corresponding authors. E-mail addresses: [email protected] (D. Nunes), [email protected] (E. Fortunato), [email protected] (R. Martins).
the systematic structural and electronic study at the nanoscale of individual metallic nanowires and their oxidized forms is imperative, giving the idea of charge behavior among the Cu-based materials. Structural and electronic properties of nanowires can be investigated at the nanoscale using atomic force microscopy (AFM) techniques [25–31], such as electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM) [25]. Charge mapping by EFM provides information on stored charge and charge transport at atmospheric pressure [32,33], while KPFM can be used to measure surface potential differences [32]. Moreover, electric interactions between the nanowire and the substrate, namely dielectric, require additional scrutiny. Few studies of these interactions with EFM and KPFM for different sources of materials have been reported [25,33,34], however exploring the behavior of Cu and Cu-oxide nanowires has not been reported so far. The present work reports the charge distribution and surface potential of different types of Cu-based nanowires (Cu, Cu2O and CuO) measured by EFM and KPFM, respectively. Structural characterization of the metallic and oxide-based nanowires has been carried out by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). 2. Experimental procedure Copper nanowires have been synthesized via a wet chemistry route following the procedure described in Ref. [1]. Sigma-Aldrich reagents were used as received and without further purification. The as-
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Please cite this article as: D. Nunes, et al., Charging effects and surface potential variations of Cu-based nanowires, Thin Solid Films (2015), http:// dx.doi.org/10.1016/j.tsf.2015.11.077
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synthesized Cu nanowires were subsequently sonicated in de-ionized water or isopropyl alcohol (IPA) for 2 min, using an ultrasonic probe (40% power) to promote a homogeneous dispersion of the nanowires before the oxidation experiments. Microwave oxidation of Cu to Cu2O nanowires was performed in a CEM Focused Microwave Synthesis System Discover SP operated at 200 W for 45 min, with a temperature of approximately 150 °C, a maximum pressure of 1.7 × 106 Pa and constant stirring, as described in Ref. [1]. This configuration prevented the evaporation of the dispersion medium (wet oxidation). Thermal oxidation of the Cu to CuO nanowires has been performed in a Nabertherm furnace under atmospheric conditions; the Cu nanowire dispersion was poured into an open quartz vessel to allow evaporation of the dispersion medium (dry oxidation) during the heat treatment at 300 °C for 24 h [17]. Dispersion media were selected as a function of their boiling temperature (TH2O = 100 °C and TIPA = 82.4 °C [35,36]); IPA was employed in furnace oxidation due to its faster evaporation, while water was employed for microwave oxidation to insure a higher reaction temperature [1,23]. X-ray diffraction has been performed with a PANalytical's X'Pert PRO MPD diffractometer using CuKα radiation. The nanowire dispersions have been dried in vacuum for 24 h before the XRD measurements. The XRD data have been acquired in the 25–65° 2θ range with a step size of 0.016°. For comparison Cu, CuO and Cu2O powder diffractograms have been simulated with PowderCell [37] using crystallographic data from Ref. [38]. SEM observations were carried out using a Carl Zeiss AURIGA CrossBeam (FIB–SEM) workstation. SEM sample preparation involved depositing a drop of the sonicated dispersion onto a metallic sample holder. Transmission electron microscopy was carried out using a Hitachi H8100 instrument operated at 200 kV and a FEI Titan G2 60-300 probe corrected microscope equipped with a Bruker EDS detector and operated at 300 kV. A drop of sonicated dispersion was deposited onto a 200-mesh copper grid covered with formvar and allowed to dry before observation. For the AFM experiments, the nanowires were dispersed in IPA and several drops of the sonicated dispersion were deposited onto a SiO2 thin film with a thickness of 100 nm on top of a Si wafer and allowed to dry. Interdigitated Mo contacts were patterned by conventional photo-lithography protocols and connected to a molybdenum film of 100 nm sputtered using an AJA ATC-1300 radio frequency magnetron sputtering system with a 3″ Mo target in Ar atmosphere, followed by lift-off procedures. AFM measurements were carried out in air at room temperature with an MFP-3D stand alone Asylum Research instrument in tapping mode using Olympus AC240 TM platinum coated silicon probes attached to cantilevers with quality factor (Q) of 116 and nominal spring constant (k) of 2 N/m. The scanning range varied from 2 × 2 to 5 × 5 μm2 with a resolution of 256 by 256 lines in all acquisition channels for both the EFM and KPFM experiments. EFM and KPFM are based on electrostatic interactions with longrange character [39–42] and in both techniques image acquisition is carried out in a two-pass sequence, with the first scan, in tapping mode, originating a topographic image that is used as height reference. In the second pass the probe is lifted above the sample and re-scans the surface following the previously recorded topography at a constant probe–sample separation. In the case of EFM, during the second pass a DC bias is applied between the conductive probe and the sample surface while the cantilever is driven mechanically by the tapping piezo. Any electrostatic force gradients present at the sample surface shift the resonance frequency of the probe, changing both the amplitude and phase of the cantilever oscillation. These changes are a qualitative measure of capacitive gradients on the sample surface [43]. In KPFM, the probe is not driven by the tapping piezo during the second pass; rather an AC voltage is applied to the probe at its resonance frequency. In these conditions, when the scanning probe senses a DC potential at the sample surface the
cantilever oscillates and a feedback loop is used to restore the probe–sample separation. The restoring DC voltage applied to the probe can be used to map quantitatively the surface potential at a resolution of a few nanometers, yielding information on the local composition and electronic state of the nanowires [44]. During the EFM measurements the nanowires were grounded through the Mo interdigitated contacts to allow the injection of charges into the nanowire and the sputtered Mo film acted as a potential sink. Fig. 1 illustrates the setup of the EFM measurement system. The underlying SiO2 dielectric layer and surrounding atmosphere prevented charge drain out from the nanowire surface, i.e., in the presence of the applied bias, mobile charges could only flow along the nanowire to or from the metal contact. In EFM the phase shifts of the cantilever oscillation are related to the gradients of electrostatic force (F) with respect to the probe/sample separation (δ) by [33,40,43]: ϕ ¼ tan−1
Q ′ F k
ð1Þ
with: F0 ¼
2
∂F 1∂ C 2 V ¼ ∂δ 2 ∂δ2 ps
ð2Þ
where C is the capacitance between the probe and the sample and Vps is the applied bias. The convention in EFM for the phase shift is ϕ-π/2, which is known as Φ signal. Therefore, the relation between phase shift and applied bias can be computed as [33]: 2
Φ¼
Q ∂F Q ∂ C 2 V ¼ k ∂δ 2k ∂δ2 ps
ð3Þ
At low lift heights the EFM contrast increases [45], however, since in these conditions detrimental electrostatic effects from the dielectric substrate were expected, optimization of the probe–sample separation was required. The second pass of the EFM measurements was carried out at a lift height of 50 and 100 nm with a bias of +1, +5 or +10 V, with the latter being the limit of the system. The scan rate was 1 Hz. Charge distribution profiles were obtained using the IGOR Pro 6.22A data analysis software. In KPFM when the probe is brought close to the sample surface their Fermi energy levels tend to align through electron tunneling making both surfaces charged. This will generate an apparent potential
Fig. 1. Scheme of the EFM measurement setup showing a single nanowire attached to the Mo interdigitated contact.
Please cite this article as: D. Nunes, et al., Charging effects and surface potential variations of Cu-based nanowires, Thin Solid Films (2015), http:// dx.doi.org/10.1016/j.tsf.2015.11.077
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3. Results and discussion 3.1. X-Ray diffraction Fig. 2 shows the XRD diffractograms of the nanowires before and after oxidation for both oxidizing routes. Single phase Cu2O is not easily attainable with thermal oxidation in air [17,19,46], since complete elimination of metallic Cu is usually associated with a prominent fraction of CuO. Therefore, thermal oxidation was used to produce CuO nanowires while microwave oxidation was employed to guarantee a major presence of Cu2O [1]. As can be seen in Fig. 2, the peaks of the initial nanowires could be fully assigned to metallic copper, while microwave oxidation revealed a nearly full conversion of metallic copper into Cu2O with minor CuO formation [1]. Furnace annealing resulted essentially in CuO with only traces of Cu2O. The XRD peak widths show that the pure Cu nanowires and the Cu2O nanowires grown by microwave oxidation had larger crystallite size than the CuO nanowires obtained by furnace annealing (Fig. 2) [1]. Fig. 2. XRD diffractograms of the as-synthesized nanowires after oxidation under microwave irradiation [1] and furnace annealing in air. Simulated diffractograms are shown below for comparison.
difference between the two surfaces, the so called contact potential difference (VCPD), proportional to the work function (WF) difference [42]:
V CPD ¼
φp −φs e
ð4Þ
where φp and φs are the work functions of probe and sample, respectively, and e is the electronic charge. The electrical force acting on the contact area due to VCPD induces cantilever oscillations and the effect can be nullified by a restoring DC bias (VDC) applied to the probe through a feedback loop locked on amplitude. These VDC values are used to map surface potential variations on the sample. The second pass of the KPFM measurements was performed at an optimized height of 50 nm above the topographic trace while applying an AC voltage of 5 V to the probe. A reference of pure gold disk has been used for calibration of the probe work function. For the EFM and KPFM measurements, at least three individual nanowires for each condition have been analyzed.
3.2. Electron microscopy The morphology of the nanowires is shown in Fig. 3. The relatively smooth surface of the initial Cu nanowires (Fig. 3 (a) and (d)) was roughened during oxidation, which also resulted in: i) shorter nanowires due to strain generated during oxidation [19] that lead to fracture, and ii) higher diameters arising from the incorporation of oxygen [1,19] (Fig. 3 (b) and (c)). The results in Fig. 3 (b) and (e) are in line with the ones reported in Ref. [1] for Cu2O nanowires grown by microwave oxidation, with large polyhedral crystals aligned along the nanowire body (widths of 170 ± 90 nm and lengths of 485 ± 279 nm [1]). In contrast, annealing in air produced CuO nanocrystals of about 30 nm agglomerated into hollow nanowires (Fig. 3 (c) and (f)). These results are in agreement with the XRD peak widths in Fig. 2. The oxidation into CuO during annealing in air is based on solid–gas reactions involving inward diffusion of oxygen and outward diffusion of Cu [19,47]. The metallic Cu reacts with oxygen to form Cu2O nanocrystals on the nanowire surface and the oxide layer grows both inwards and outwards [19]. The progression of the oxidation reaction leads to the appearance of CuO on the periphery of the Cu2O layer that grows consuming the Cu2O phase [47]. The hollow configuration (Fig. 3 (c)) is attributed to the rapid outward diffusion of copper, which is not balanced by the internal oxide formation. Diffusion of Cu
Fig. 3. SEM and TEM images of nanowires (a and d) Cu, (b and e) Cu2O produced by microwave irradiation, and (c and f) CuO nanowires oxidized by furnace annealing in air.
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Fig. 4. TEM images of a Cu nanowire revealing the presence of stacking faults (a) and native oxide protrusions (b). STEM image (c) and overlapping of the corresponding Cu and O X-ray maps with quantitative relative contrast (d) and individually optimized contrast (e).
Fig. 5. (a) Topography and EFM images of a Cu nanowire for a Vps of (b) 0 V, (c) +1 V (d) +5 V and (e) +10 V at a lift height of 100 nm.
Please cite this article as: D. Nunes, et al., Charging effects and surface potential variations of Cu-based nanowires, Thin Solid Films (2015), http:// dx.doi.org/10.1016/j.tsf.2015.11.077
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Fig. 6. (a) Phase profile along the Cu nanowire for different Vps (Fig. 5). The profiles were averaged across a width of 60 nm at the center of the nanowire. (b) Average phase shift at the nanowire as a function of the applied bias. A Φ ∝ V2ps relation was used to fit the experimental results.
is favored by the large number of vacancies arising from the high surface-to-volume ratio of the nanostructure. Similar hollow structures have been proposed to result from a Kirkendall-type effect [48] based on the non-equilibrium mutual diffusion at different rates and a concomitant coalescence of vacancies on the side of the fast diffusion species [49]. Additional features of the Cu nanowires are presented in Fig. 4. Structural defects such as stacking faults were typically found along the length of the nanowires (Fig. 4 (a)). Furthermore, surface oxide protrusions were consistently detected (Fig. 4 (b) and (e)). The occurrence of native Cu2O on the surface of Cu nanowires synthesized via wet chemistry routes is expected [50], nevertheless, a minor presence of oxygen can be inferred from the overlapped Cu and O maps with quantitative relative contrast (Fig. 4 (d)).
3.3. Electrostatic force microscopy and Kelvin probe microscopy A topographic image and corresponding EFM maps of a single Cu nanowire obtained with different bias are presented in Fig. 5. The average phase variation at the nanowire increased with applied bias as clearly evidenced by the profiles taken along the nanowire and shown in Fig. 6 (a). Localized phase shifts were significantly enhanced by the higher external fields (Fig. 6 (a)). The topographic image and the color map presented in Fig. 7 demonstrate an association between these phase changes and the presence of oxide protrusions (Figs. 4 and 7 (c)) [1,51]. Therefore, the phase dips observed for Vps = + 5 V and +10 V originate from charge trapping mainly around oxide protrusions and possibly on structural defects (Figs. 4 and 7), which induced changes in the local potential energy. This non-uniform charge distribution is expected to introduce localized semiconducting behavior in the metallic nanowires [52]. The average phase shift (Φ) of each profile in Fig. 6 (a) has been plotted against Vps in Fig. 6 (b) and the Φ ∝ V2ps theoretical relation, describing the capacitance coupling between probe and
nanowire (Eq. (3)), has been fitted to the experimental results. The experimental deviation at higher Vps values results from the contribution of the substrate polarization to the capacitive system. The slight phase difference between the Cu nanowire and the SiO2 substrate at Vps = 0 V (Fig. 5 (b)) originates from the residual electrostatic charges in the substrate [53]. On the other hand, the bright edges that envelop the nanowire at Vps ≠ 0 resulted from the polarization of the dielectric substrate by the negatively charged nanowire, an effect that increased with applied bias (see Figs. 5 and 6). As shown by the surface plot of the EFM map obtained at Vps = +10 V (Fig. 8), the width of the polarization envelope decreased with the distance from the Mo contact. Since, for a given bias, the average charge at the nanowire did not significantly decrease with the distance from the contact (see Fig. 6 (a)), the decreasing width of the polarization envelopes seems related to opposite charge transfers from/to the ground, i.e., (i) negative charges were injected from the ground into copper to produce the probe-nanowire capacitance effect, while (ii) the dielectric substrate was assisted by the ground in compensating the electrostatic field around the charged nanowire, with stronger transfer occurring nearer the Mo contact, particularly, for higher bias. A similar investigation on localized electrostatic field distribution has been carried out for the Cu oxide nanowires, i.e Cu2O and CuO. A comparison between EFM images of Cu2O and CuO with different applied bias (Vps = 0 V, + 1 V, + 5 V and + 10 V) is presented in Fig. 9. A slight phase difference between the nanowires and the SiO2 substrate was detected without application of an external field (Fig. 9 (b) and (g)). This effect is expected to originate from residual electrostatic charges in the substrate. Upon application of an external field of Vps = + 1 V, a weaker phase difference was generated between Cu2O and SiO2 than between CuO and SiO2 (Fig. 9 (c) and (h)). This phase difference increased sharply with Vps = +5 V and +10 V, as observed for the Cu nanowires. The charge in the Cu2O and CuO nanowires induced the buildup of a polarization envelope similar to the ones observed for the Cu nanowires (Fig. 9 (d), (e), (i) and (j)).
Fig. 7. (a) Topography image and (b) color map of the corresponding EFM image obtained at +10 V. (c) TEM image showing native oxide protrusions and stacking faults.
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Fig. 8. Surface plot of the EFM image at +10 V (Fig. 5 (e)). The decrease of the envelope width with the distance to the Mo contact is evident.
Fig. 10 shows phase shift profiles taken along the nanowires and their average phase shift (Φ) against applied bias for the Cu oxide nanowires. Fig. 10 (a) and (b) evidences the presence of phase dips for Vps =
+5 V and +10 V such as in Cu nanowires. Thus, this behavior could also be related to charge trapping along the nanowires. The average phase shift is higher for CuO compared to Cu2O for all applied bias (Fig. 10 (c))and a stronger contrast was also observed in CuO EFM images (compare Fig. 9 (b) to (e) with (g) to (j)). This dissimilar behavior can be justified by the lower band gap of CuO (1.4 eV [11]) compared to the Cu2O one (2.17 eV [1,54]). Nevertheless, at Vps of + 10 V the phase shift values are comparable indicating that the strong voltage imposed overcame the differences in electronic behavior between the materials. As for the Cu nanowires, the theoretical relation Φ ∝ V2ps is well fitted to the experimental results for both Cu oxides (Fig. 6 (b)). KPFM measurements were carried out for all the materials investigated (data presented in Fig. 11). A standard gold disk with a work function of 5.3 eV was used to calibrate the probe [55]. Fig. 11 (d) to (f) exhibited the contact potential difference (CPD) profiles for each material, where the average surface potential can be estimated to be 0.185 V, 0.02 V, and 0.93 V for Cu, Cu2O and CuO respectively. From the CPD profiles
Fig. 9. Topography of (a) Cu2O and (f) CuO nanowires at a lift height of 50 nm. The corresponding EFM images were acquired with a Vps of (b and g) 0 V, (c and h) +1 V, (d and i) +5 V, (e and j) +10 V.
Please cite this article as: D. Nunes, et al., Charging effects and surface potential variations of Cu-based nanowires, Thin Solid Films (2015), http:// dx.doi.org/10.1016/j.tsf.2015.11.077
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Fig. 10. Phase profiles along the (a) Cu2O and (b) CuO nanowires (Fig. 9) for different Vps, and (c) corresponding average phase shift as a function of the applied bias. The abrupt phase shift for Cu2O (around 1 μm) is due to a localized defect on the structure. The profiles were averaged across a width of 60 nm at the center of the nanowire. A Φ ∝ V2ps relation was used to fit the experimental results.
and Eq. 4, the work function of Cu nanowires was determined to be 5.41 eV, while for Cu2O and CuO were 5.58 and 4.67 eV, respectively. These values are in accordance with literature work function values 4.48 to 5.30 eV for Cu [56], 5.20 to 5.60 eV for Cu2O [57,58], and 4.80 eV for CuO [59]. Moreover, from Fig. 11 (d) to (e), it could also be observed
that the contact potential differences between the substrate (SiO2) and Cu or Cu2O nanowires are negative, whereas for CuO is positive (Fig. 11 (f)), which can be well associated to the differences in work functions of each material to the work function of SiO2 (4.52 eV [60]). In terms of surface potential it may be concluded that WFCuO N WFCu N WFCu2O.
Fig. 11. Surface potential images of a Cu (a), Cu2O (b) and CuO (c) nanowires obtained from KPFM measurements. Topography images of each nanowire are presented as insets. The CPD profiles from images (a) to (c) are presented from (d) to (f). The profiles were averaged across a width of 20 nm along the diameter of the nanowire (see the arrows). An anomalous effect is observed on the metal edge in (c) associated to topographic effects.
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4. Conclusions The Cu nanowires were oxidized under microwave irradiation or by furnace annealing in air, originating Cu2O and CuO nanowires, respectively, with remarkable structural differences. The electronic characterization of Cu-based single nanowires yielded relative contact potentials in line with literature values for the respective work functions: WFCuO N WFCu N WFCu2O. High external fields resulted in the buildup of polarization envelopes in the SiO2 substrate around the charged nanowires. The Cu nanowires displayed a variable distribution of charges along their length in association with surface/structural defects and native oxide protrusions, which altered the local potential energy level. Therefore, the capacitance effect between the nanowire and the substrate was locally disturbed by the nanowire structural/surface defects, an effect worth exploring in future research as the localized charge distribution is relevant for the integration of these materials in single nanowire-based devices.
Acknowledgments The work was supported by the FCT — Portuguese Science and Technology Foundation, through the scholarship BPD/84215/2012 and the project EXCL/CTM-NAN/0201/2012, as well as by the European project CEOPS with the grant agreement no: 309984. The work was also supported by FEDER funds through the COMPETE 2020 Programme and National Funds through FCT under the project UID/CTM/50025/2013.
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Charging effects and surface potential variations of Cu-based nanowires D. Nunes a,⁎, T.R. Calmeiro a, S. Nandy a, J.V. Pinto a, A. Pimentel a, P. Barquinha a, P.A. Carvalho b,c, J.C. Walmsley d, E. Fortunato a,⁎, R. Martins a,⁎ a
i3N/CENIMAT, Department of Materials Science, Faculty of Science and Technology, Universidade NOVA de Lisboa, Campus de Caparica, 2829-516 Caparica, Portugal SINTEF Materials and Chemistry, PB 124 Blindern, NO-0314, Oslo, Norway CeFEMA, Instituto Superior Técnico, Universidade de Lisboa, 1049-001, Lisboa, Portugal d SINTEF Materials and Chemistry, Materials and Nanotechnology, Høgskoleringen 5, 7034 Trondheim, Norway b c
a r t i c l e
i n f o
Article history: Received 15 June 2015 Received in revised form 27 November 2015 Accepted 29 November 2015 Available online xxxx Keywords: Cu-based nanowires Electrostatic force microscopy Kelvin probe force microscopy, charge mapping Dielectric polarization Capacitive effects
a b s t r a c t The present work reports charging effects and surface potential variations in pure copper, cuprous oxide and cupric oxide nanowires observed by electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM). The copper nanowires were produced by wet synthesis, oxidation into cuprous oxide nanowires was achieved through microwave irradiation and cupric oxide nanowires were obtained via furnace annealing in atmospheric conditions. Structural characterization of the nanowires was carried out by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. During the EFM experiments the electrostatic field of the positive probe charged negatively the Cu-based nanowires, which in turn polarized the SiO2 dielectric substrate. Both the probe/nanowire capacitance as well as the substrate polarization increased with the applied bias. Cu2O and CuO nanowires behaved distinctively during the EFM measurements in accordance with their band gap energies. The work functions (WF) of the Cu-based nanowires, obtained by KPFM measurements, yielded WFCuO N WFCu N WFCu2O. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Copper-based nanowires have attracted a growing interest in advanced materials and nanotechnology research [1–3] owing to the high electrical conductivity of copper [4] and the p-type semiconductor behavior of both cuprous oxide (Cu2O) [1,5–8] and cupric oxide (CuO) [9,10], which present energy band gaps of, respectively, 2.17 eV [1] and 1.4 eV [11]. These materials are interesting for a plethora of nanooptoelectronic applications [12–14], ranging from solar cells [15], field effect transistors [12], transparent flexible electrodes integrated in capacitive touch sensors [16] to glucose [10] and gas sensors [9]. The oxidation of Cu nanowires has been extensively studied over the years [1,17–19], in which the thermal route is usually adopted due to its simplicity and high-quality/low-cost relation [20,21]. Although the method is effective for the production of CuO nanowires, high fractions of Cu2O nanowires are difficult to achieve. An alternative route based on microwave irradiation [22–24] has proven to be capable of producing nearly pure Cu2O nanowires with suitable efficiency/cost balance [1]. Nevertheless, when device integration is sought, the final performance will depend critically on the nanowire characteristics and a reproducible performance demands uniform electrical properties along each nanowire as well as from wire to wire in the same batch, which remains challenging. Thus, ⁎ Corresponding authors. E-mail addresses: [email protected] (D. Nunes), [email protected] (E. Fortunato), [email protected] (R. Martins).
the systematic structural and electronic study at the nanoscale of individual metallic nanowires and their oxidized forms is imperative, giving the idea of charge behavior among the Cu-based materials. Structural and electronic properties of nanowires can be investigated at the nanoscale using atomic force microscopy (AFM) techniques [25–31], such as electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM) [25]. Charge mapping by EFM provides information on stored charge and charge transport at atmospheric pressure [32,33], while KPFM can be used to measure surface potential differences [32]. Moreover, electric interactions between the nanowire and the substrate, namely dielectric, require additional scrutiny. Few studies of these interactions with EFM and KPFM for different sources of materials have been reported [25,33,34], however exploring the behavior of Cu and Cu-oxide nanowires has not been reported so far. The present work reports the charge distribution and surface potential of different types of Cu-based nanowires (Cu, Cu2O and CuO) measured by EFM and KPFM, respectively. Structural characterization of the metallic and oxide-based nanowires has been carried out by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). 2. Experimental procedure Copper nanowires have been synthesized via a wet chemistry route following the procedure described in Ref. [1]. Sigma-Aldrich reagents were used as received and without further purification. The as-
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D. Nunes et al. / Thin Solid Films xxx (2015) xxx–xxx
synthesized Cu nanowires were subsequently sonicated in de-ionized water or isopropyl alcohol (IPA) for 2 min, using an ultrasonic probe (40% power) to promote a homogeneous dispersion of the nanowires before the oxidation experiments. Microwave oxidation of Cu to Cu2O nanowires was performed in a CEM Focused Microwave Synthesis System Discover SP operated at 200 W for 45 min, with a temperature of approximately 150 °C, a maximum pressure of 1.7 × 106 Pa and constant stirring, as described in Ref. [1]. This configuration prevented the evaporation of the dispersion medium (wet oxidation). Thermal oxidation of the Cu to CuO nanowires has been performed in a Nabertherm furnace under atmospheric conditions; the Cu nanowire dispersion was poured into an open quartz vessel to allow evaporation of the dispersion medium (dry oxidation) during the heat treatment at 300 °C for 24 h [17]. Dispersion media were selected as a function of their boiling temperature (TH2O = 100 °C and TIPA = 82.4 °C [35,36]); IPA was employed in furnace oxidation due to its faster evaporation, while water was employed for microwave oxidation to insure a higher reaction temperature [1,23]. X-ray diffraction has been performed with a PANalytical's X'Pert PRO MPD diffractometer using CuKα radiation. The nanowire dispersions have been dried in vacuum for 24 h before the XRD measurements. The XRD data have been acquired in the 25–65° 2θ range with a step size of 0.016°. For comparison Cu, CuO and Cu2O powder diffractograms have been simulated with PowderCell [37] using crystallographic data from Ref. [38]. SEM observations were carried out using a Carl Zeiss AURIGA CrossBeam (FIB–SEM) workstation. SEM sample preparation involved depositing a drop of the sonicated dispersion onto a metallic sample holder. Transmission electron microscopy was carried out using a Hitachi H8100 instrument operated at 200 kV and a FEI Titan G2 60-300 probe corrected microscope equipped with a Bruker EDS detector and operated at 300 kV. A drop of sonicated dispersion was deposited onto a 200-mesh copper grid covered with formvar and allowed to dry before observation. For the AFM experiments, the nanowires were dispersed in IPA and several drops of the sonicated dispersion were deposited onto a SiO2 thin film with a thickness of 100 nm on top of a Si wafer and allowed to dry. Interdigitated Mo contacts were patterned by conventional photo-lithography protocols and connected to a molybdenum film of 100 nm sputtered using an AJA ATC-1300 radio frequency magnetron sputtering system with a 3″ Mo target in Ar atmosphere, followed by lift-off procedures. AFM measurements were carried out in air at room temperature with an MFP-3D stand alone Asylum Research instrument in tapping mode using Olympus AC240 TM platinum coated silicon probes attached to cantilevers with quality factor (Q) of 116 and nominal spring constant (k) of 2 N/m. The scanning range varied from 2 × 2 to 5 × 5 μm2 with a resolution of 256 by 256 lines in all acquisition channels for both the EFM and KPFM experiments. EFM and KPFM are based on electrostatic interactions with longrange character [39–42] and in both techniques image acquisition is carried out in a two-pass sequence, with the first scan, in tapping mode, originating a topographic image that is used as height reference. In the second pass the probe is lifted above the sample and re-scans the surface following the previously recorded topography at a constant probe–sample separation. In the case of EFM, during the second pass a DC bias is applied between the conductive probe and the sample surface while the cantilever is driven mechanically by the tapping piezo. Any electrostatic force gradients present at the sample surface shift the resonance frequency of the probe, changing both the amplitude and phase of the cantilever oscillation. These changes are a qualitative measure of capacitive gradients on the sample surface [43]. In KPFM, the probe is not driven by the tapping piezo during the second pass; rather an AC voltage is applied to the probe at its resonance frequency. In these conditions, when the scanning probe senses a DC potential at the sample surface the
cantilever oscillates and a feedback loop is used to restore the probe–sample separation. The restoring DC voltage applied to the probe can be used to map quantitatively the surface potential at a resolution of a few nanometers, yielding information on the local composition and electronic state of the nanowires [44]. During the EFM measurements the nanowires were grounded through the Mo interdigitated contacts to allow the injection of charges into the nanowire and the sputtered Mo film acted as a potential sink. Fig. 1 illustrates the setup of the EFM measurement system. The underlying SiO2 dielectric layer and surrounding atmosphere prevented charge drain out from the nanowire surface, i.e., in the presence of the applied bias, mobile charges could only flow along the nanowire to or from the metal contact. In EFM the phase shifts of the cantilever oscillation are related to the gradients of electrostatic force (F) with respect to the probe/sample separation (δ) by [33,40,43]: ϕ ¼ tan−1
Q ′ F k
ð1Þ
with: F0 ¼
2
∂F 1∂ C 2 V ¼ ∂δ 2 ∂δ2 ps
ð2Þ
where C is the capacitance between the probe and the sample and Vps is the applied bias. The convention in EFM for the phase shift is ϕ-π/2, which is known as Φ signal. Therefore, the relation between phase shift and applied bias can be computed as [33]: 2
Φ¼
Q ∂F Q ∂ C 2 V ¼ k ∂δ 2k ∂δ2 ps
ð3Þ
At low lift heights the EFM contrast increases [45], however, since in these conditions detrimental electrostatic effects from the dielectric substrate were expected, optimization of the probe–sample separation was required. The second pass of the EFM measurements was carried out at a lift height of 50 and 100 nm with a bias of +1, +5 or +10 V, with the latter being the limit of the system. The scan rate was 1 Hz. Charge distribution profiles were obtained using the IGOR Pro 6.22A data analysis software. In KPFM when the probe is brought close to the sample surface their Fermi energy levels tend to align through electron tunneling making both surfaces charged. This will generate an apparent potential
Fig. 1. Scheme of the EFM measurement setup showing a single nanowire attached to the Mo interdigitated contact.
Please cite this article as: D. Nunes, et al., Charging effects and surface potential variations of Cu-based nanowires, Thin Solid Films (2015), http:// dx.doi.org/10.1016/j.tsf.2015.11.077
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3. Results and discussion 3.1. X-Ray diffraction Fig. 2 shows the XRD diffractograms of the nanowires before and after oxidation for both oxidizing routes. Single phase Cu2O is not easily attainable with thermal oxidation in air [17,19,46], since complete elimination of metallic Cu is usually associated with a prominent fraction of CuO. Therefore, thermal oxidation was used to produce CuO nanowires while microwave oxidation was employed to guarantee a major presence of Cu2O [1]. As can be seen in Fig. 2, the peaks of the initial nanowires could be fully assigned to metallic copper, while microwave oxidation revealed a nearly full conversion of metallic copper into Cu2O with minor CuO formation [1]. Furnace annealing resulted essentially in CuO with only traces of Cu2O. The XRD peak widths show that the pure Cu nanowires and the Cu2O nanowires grown by microwave oxidation had larger crystallite size than the CuO nanowires obtained by furnace annealing (Fig. 2) [1]. Fig. 2. XRD diffractograms of the as-synthesized nanowires after oxidation under microwave irradiation [1] and furnace annealing in air. Simulated diffractograms are shown below for comparison.
difference between the two surfaces, the so called contact potential difference (VCPD), proportional to the work function (WF) difference [42]:
V CPD ¼
φp −φs e
ð4Þ
where φp and φs are the work functions of probe and sample, respectively, and e is the electronic charge. The electrical force acting on the contact area due to VCPD induces cantilever oscillations and the effect can be nullified by a restoring DC bias (VDC) applied to the probe through a feedback loop locked on amplitude. These VDC values are used to map surface potential variations on the sample. The second pass of the KPFM measurements was performed at an optimized height of 50 nm above the topographic trace while applying an AC voltage of 5 V to the probe. A reference of pure gold disk has been used for calibration of the probe work function. For the EFM and KPFM measurements, at least three individual nanowires for each condition have been analyzed.
3.2. Electron microscopy The morphology of the nanowires is shown in Fig. 3. The relatively smooth surface of the initial Cu nanowires (Fig. 3 (a) and (d)) was roughened during oxidation, which also resulted in: i) shorter nanowires due to strain generated during oxidation [19] that lead to fracture, and ii) higher diameters arising from the incorporation of oxygen [1,19] (Fig. 3 (b) and (c)). The results in Fig. 3 (b) and (e) are in line with the ones reported in Ref. [1] for Cu2O nanowires grown by microwave oxidation, with large polyhedral crystals aligned along the nanowire body (widths of 170 ± 90 nm and lengths of 485 ± 279 nm [1]). In contrast, annealing in air produced CuO nanocrystals of about 30 nm agglomerated into hollow nanowires (Fig. 3 (c) and (f)). These results are in agreement with the XRD peak widths in Fig. 2. The oxidation into CuO during annealing in air is based on solid–gas reactions involving inward diffusion of oxygen and outward diffusion of Cu [19,47]. The metallic Cu reacts with oxygen to form Cu2O nanocrystals on the nanowire surface and the oxide layer grows both inwards and outwards [19]. The progression of the oxidation reaction leads to the appearance of CuO on the periphery of the Cu2O layer that grows consuming the Cu2O phase [47]. The hollow configuration (Fig. 3 (c)) is attributed to the rapid outward diffusion of copper, which is not balanced by the internal oxide formation. Diffusion of Cu
Fig. 3. SEM and TEM images of nanowires (a and d) Cu, (b and e) Cu2O produced by microwave irradiation, and (c and f) CuO nanowires oxidized by furnace annealing in air.
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Fig. 4. TEM images of a Cu nanowire revealing the presence of stacking faults (a) and native oxide protrusions (b). STEM image (c) and overlapping of the corresponding Cu and O X-ray maps with quantitative relative contrast (d) and individually optimized contrast (e).
Fig. 5. (a) Topography and EFM images of a Cu nanowire for a Vps of (b) 0 V, (c) +1 V (d) +5 V and (e) +10 V at a lift height of 100 nm.
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Fig. 6. (a) Phase profile along the Cu nanowire for different Vps (Fig. 5). The profiles were averaged across a width of 60 nm at the center of the nanowire. (b) Average phase shift at the nanowire as a function of the applied bias. A Φ ∝ V2ps relation was used to fit the experimental results.
is favored by the large number of vacancies arising from the high surface-to-volume ratio of the nanostructure. Similar hollow structures have been proposed to result from a Kirkendall-type effect [48] based on the non-equilibrium mutual diffusion at different rates and a concomitant coalescence of vacancies on the side of the fast diffusion species [49]. Additional features of the Cu nanowires are presented in Fig. 4. Structural defects such as stacking faults were typically found along the length of the nanowires (Fig. 4 (a)). Furthermore, surface oxide protrusions were consistently detected (Fig. 4 (b) and (e)). The occurrence of native Cu2O on the surface of Cu nanowires synthesized via wet chemistry routes is expected [50], nevertheless, a minor presence of oxygen can be inferred from the overlapped Cu and O maps with quantitative relative contrast (Fig. 4 (d)).
3.3. Electrostatic force microscopy and Kelvin probe microscopy A topographic image and corresponding EFM maps of a single Cu nanowire obtained with different bias are presented in Fig. 5. The average phase variation at the nanowire increased with applied bias as clearly evidenced by the profiles taken along the nanowire and shown in Fig. 6 (a). Localized phase shifts were significantly enhanced by the higher external fields (Fig. 6 (a)). The topographic image and the color map presented in Fig. 7 demonstrate an association between these phase changes and the presence of oxide protrusions (Figs. 4 and 7 (c)) [1,51]. Therefore, the phase dips observed for Vps = + 5 V and +10 V originate from charge trapping mainly around oxide protrusions and possibly on structural defects (Figs. 4 and 7), which induced changes in the local potential energy. This non-uniform charge distribution is expected to introduce localized semiconducting behavior in the metallic nanowires [52]. The average phase shift (Φ) of each profile in Fig. 6 (a) has been plotted against Vps in Fig. 6 (b) and the Φ ∝ V2ps theoretical relation, describing the capacitance coupling between probe and
nanowire (Eq. (3)), has been fitted to the experimental results. The experimental deviation at higher Vps values results from the contribution of the substrate polarization to the capacitive system. The slight phase difference between the Cu nanowire and the SiO2 substrate at Vps = 0 V (Fig. 5 (b)) originates from the residual electrostatic charges in the substrate [53]. On the other hand, the bright edges that envelop the nanowire at Vps ≠ 0 resulted from the polarization of the dielectric substrate by the negatively charged nanowire, an effect that increased with applied bias (see Figs. 5 and 6). As shown by the surface plot of the EFM map obtained at Vps = +10 V (Fig. 8), the width of the polarization envelope decreased with the distance from the Mo contact. Since, for a given bias, the average charge at the nanowire did not significantly decrease with the distance from the contact (see Fig. 6 (a)), the decreasing width of the polarization envelopes seems related to opposite charge transfers from/to the ground, i.e., (i) negative charges were injected from the ground into copper to produce the probe-nanowire capacitance effect, while (ii) the dielectric substrate was assisted by the ground in compensating the electrostatic field around the charged nanowire, with stronger transfer occurring nearer the Mo contact, particularly, for higher bias. A similar investigation on localized electrostatic field distribution has been carried out for the Cu oxide nanowires, i.e Cu2O and CuO. A comparison between EFM images of Cu2O and CuO with different applied bias (Vps = 0 V, + 1 V, + 5 V and + 10 V) is presented in Fig. 9. A slight phase difference between the nanowires and the SiO2 substrate was detected without application of an external field (Fig. 9 (b) and (g)). This effect is expected to originate from residual electrostatic charges in the substrate. Upon application of an external field of Vps = + 1 V, a weaker phase difference was generated between Cu2O and SiO2 than between CuO and SiO2 (Fig. 9 (c) and (h)). This phase difference increased sharply with Vps = +5 V and +10 V, as observed for the Cu nanowires. The charge in the Cu2O and CuO nanowires induced the buildup of a polarization envelope similar to the ones observed for the Cu nanowires (Fig. 9 (d), (e), (i) and (j)).
Fig. 7. (a) Topography image and (b) color map of the corresponding EFM image obtained at +10 V. (c) TEM image showing native oxide protrusions and stacking faults.
Please cite this article as: D. Nunes, et al., Charging effects and surface potential variations of Cu-based nanowires, Thin Solid Films (2015), http:// dx.doi.org/10.1016/j.tsf.2015.11.077
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Fig. 8. Surface plot of the EFM image at +10 V (Fig. 5 (e)). The decrease of the envelope width with the distance to the Mo contact is evident.
Fig. 10 shows phase shift profiles taken along the nanowires and their average phase shift (Φ) against applied bias for the Cu oxide nanowires. Fig. 10 (a) and (b) evidences the presence of phase dips for Vps =
+5 V and +10 V such as in Cu nanowires. Thus, this behavior could also be related to charge trapping along the nanowires. The average phase shift is higher for CuO compared to Cu2O for all applied bias (Fig. 10 (c))and a stronger contrast was also observed in CuO EFM images (compare Fig. 9 (b) to (e) with (g) to (j)). This dissimilar behavior can be justified by the lower band gap of CuO (1.4 eV [11]) compared to the Cu2O one (2.17 eV [1,54]). Nevertheless, at Vps of + 10 V the phase shift values are comparable indicating that the strong voltage imposed overcame the differences in electronic behavior between the materials. As for the Cu nanowires, the theoretical relation Φ ∝ V2ps is well fitted to the experimental results for both Cu oxides (Fig. 6 (b)). KPFM measurements were carried out for all the materials investigated (data presented in Fig. 11). A standard gold disk with a work function of 5.3 eV was used to calibrate the probe [55]. Fig. 11 (d) to (f) exhibited the contact potential difference (CPD) profiles for each material, where the average surface potential can be estimated to be 0.185 V, 0.02 V, and 0.93 V for Cu, Cu2O and CuO respectively. From the CPD profiles
Fig. 9. Topography of (a) Cu2O and (f) CuO nanowires at a lift height of 50 nm. The corresponding EFM images were acquired with a Vps of (b and g) 0 V, (c and h) +1 V, (d and i) +5 V, (e and j) +10 V.
Please cite this article as: D. Nunes, et al., Charging effects and surface potential variations of Cu-based nanowires, Thin Solid Films (2015), http:// dx.doi.org/10.1016/j.tsf.2015.11.077
D. Nunes et al. / Thin Solid Films xxx (2015) xxx–xxx
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Fig. 10. Phase profiles along the (a) Cu2O and (b) CuO nanowires (Fig. 9) for different Vps, and (c) corresponding average phase shift as a function of the applied bias. The abrupt phase shift for Cu2O (around 1 μm) is due to a localized defect on the structure. The profiles were averaged across a width of 60 nm at the center of the nanowire. A Φ ∝ V2ps relation was used to fit the experimental results.
and Eq. 4, the work function of Cu nanowires was determined to be 5.41 eV, while for Cu2O and CuO were 5.58 and 4.67 eV, respectively. These values are in accordance with literature work function values 4.48 to 5.30 eV for Cu [56], 5.20 to 5.60 eV for Cu2O [57,58], and 4.80 eV for CuO [59]. Moreover, from Fig. 11 (d) to (e), it could also be observed
that the contact potential differences between the substrate (SiO2) and Cu or Cu2O nanowires are negative, whereas for CuO is positive (Fig. 11 (f)), which can be well associated to the differences in work functions of each material to the work function of SiO2 (4.52 eV [60]). In terms of surface potential it may be concluded that WFCuO N WFCu N WFCu2O.
Fig. 11. Surface potential images of a Cu (a), Cu2O (b) and CuO (c) nanowires obtained from KPFM measurements. Topography images of each nanowire are presented as insets. The CPD profiles from images (a) to (c) are presented from (d) to (f). The profiles were averaged across a width of 20 nm along the diameter of the nanowire (see the arrows). An anomalous effect is observed on the metal edge in (c) associated to topographic effects.
Please cite this article as: D. Nunes, et al., Charging effects and surface potential variations of Cu-based nanowires, Thin Solid Films (2015), http:// dx.doi.org/10.1016/j.tsf.2015.11.077
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4. Conclusions The Cu nanowires were oxidized under microwave irradiation or by furnace annealing in air, originating Cu2O and CuO nanowires, respectively, with remarkable structural differences. The electronic characterization of Cu-based single nanowires yielded relative contact potentials in line with literature values for the respective work functions: WFCuO N WFCu N WFCu2O. High external fields resulted in the buildup of polarization envelopes in the SiO2 substrate around the charged nanowires. The Cu nanowires displayed a variable distribution of charges along their length in association with surface/structural defects and native oxide protrusions, which altered the local potential energy level. Therefore, the capacitance effect between the nanowire and the substrate was locally disturbed by the nanowire structural/surface defects, an effect worth exploring in future research as the localized charge distribution is relevant for the integration of these materials in single nanowire-based devices.
Acknowledgments The work was supported by the FCT — Portuguese Science and Technology Foundation, through the scholarship BPD/84215/2012 and the project EXCL/CTM-NAN/0201/2012, as well as by the European project CEOPS with the grant agreement no: 309984. The work was also supported by FEDER funds through the COMPETE 2020 Programme and National Funds through FCT under the project UID/CTM/50025/2013.
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