We study experimentally and theoretically the transport properties of Ag nanowire macrobundles in the presence of light irradiation. We have observed significant negative photoconductivity induced by the interaction between electrons and the excited surface plasmon polaritons (SPPs). As temperature T increases from 77 K to 304 K, the dark resistivity ρd without light irradiation increases linearly with T, and the resistivity change Δρ due to light irradiation decreases nonlinearly with increasing T. The current change |ΔI| due to light irradiation, which is proportional to the laser intensity, also decreases nonlinearly with increasing T. We explain well the experimental results using our proposed model with a new scattering channel due to the interaction between electrons and SPPs. Both our experimental and theoretical results reveal the novel phenomena due to the combination of photonics and electronics properties of Ag nanowires and they will be useful for scientific research, and technical applications.
© 2010 OSA
Nanoplasmonics has arisen considerable interest recently due to its promising applications in nano-optics, electronics and biosensors etc . One of the basic excitations in the relevant systems is the surface plasmon polariton (SPP), i.e., the collective oscillation of the electromagnetic field and electrons propagating along a metal-dielectric surface. It has the ability of strong confinement and control of energy in a nanoscale near metal nanostructures. In theses nanostructures, there are several elemental excitations of different natures, for instance, plasmon, exciton, soliton etc.. The interaction among theses excitations in nanoscale is of fundamental interest and leads to novel phenomena [2–4].
Though plasmonics has both the capacity of photonics and the miniaturization of electronics, most of the studies focused on the optical properties, such as surface enhanced Raman scattering in biosensors for single-molecule detection [5,6], surface plasmon resonators  and optical wave guiding beyond the diffraction limit in plasmonic waveguides , plasmon-induced fluorescence enhancement and quench [9,10]. Relatively less attention has been paid to the transport properties. It may be partially due to the reason that the inter-band transition plays a dominant role in optical processes, which has a characteristic energy scale (of a few eV) close to the bulk plasmon resonant energy. While for transport properties, the intra-band excitation of low energy near Fermi surface is important. For nanostructures, the importance of surface effect increases and SPP shows a linear dispersion relation for low energy limit, unlike the dispersion relation of bulk plasmon with a gap. Therefore it is interesting to explore the roles of SPPs in transport processes in nanostructures. Metal nanowire is such an excellent candidate for convenient and systematic study of the roles of SPPs in transport properties. Furthermore the metal nanowires are also ideal building blocks for nanoscale electronic and photonic devices [7,11,12]. Recently metal nanowires, in particular Ag nanowires (AgNWs) with well-defined dimensions, have attracted much attention [13–16] because bulk silver exhibits the highest electrical and thermal conductivities among all metals.
In this work we experimentally and theoretically explore the photoconductivity due to the interaction between SPPs and electrons on the surface of AgNWs. We should note that the heating effects may also have contribution to the photoconductivity. Fortunately, the characteristic time scales for the heating process and SPP-electron scattering process are quite different, which makes them distinguishable. Our results clearly show that the photo-induced SPPs have an appreciable influence on transportation of electrons in AgNWs.
2. Experimental details
In experiments, the length of the nanowire should be much larger than the diameter of the illuminating light spot to avoid possible influence on transport behavior of the metal nanowire caused by two electrodes and their interfaces. On the other hand, in order to generate strong photo-induced SPPs on the surface of the nanowire, its diameter is usually much less than μm scale. According to above-mentioned analysis, we have fabricated a macrolong pure metal Ag nanowire bundle (AgNWB) and measured the transport properties on a macroscopic circuit. Figure 1(a) shows an AgNWB of length about 8.0 mm and width about 300 μm, which was fabricated  by an improved solid-state ionics method. The two ends of the macrobundle were connected with conductive silver paint electrodes, and fixed to an insulating substrate. The microstructure of the macrobundle, comprising numerous AgNWs aligned in the same direction, is clearly shown in the scanning electron microscopy (SEM) image in Fig. 1(b). In Fig. 1(c), the transmission electron microscopy (TEM) image shows that the diameters of AgNWs in the macrobundle range from 30 to 90 nm.
After the fabrication of the AgNWB, we studied the influence of SPP-electron interaction on the transport behavior under continuous and pulsed laser field. We placed the sample in a cryostat at a vacuum about 10−3 Pa to control its temperature and to prevent it from being affected by air [shown in Fig. 1(d)]. An optical chopper is used for periodically interrupting a 532 nm monochromatic light beam from a semiconductor laser. The transmission coefficient of the quartz glass window of the cryostat at a wavelength of 532 nm is about 90%. In order to avoid generation of the photo-induced voltage, the laser spot is located near the geometric center of the AgNWB . The laser spot diameter on the sample is about 4.0 mm.
3. The experimental results
The I-V characteristics [shown in Fig. 2(a) ] of the AgNWB without light irradiation were recorded by a SourceMeter (Keithley 2400) at 304 K (red line) and 77 K (blue line), respectively. The results show the ohmic contacts between Ag electrode and AgNWB. The temperature dependence of dark resistance Rd of the sample measured in the temperature range 77 K to 304 K is displayed in Fig. 2(b). It clearly shows a linear relationship between the dark resistance Rd and sample temperature T.
As shown in Fig. 2(c), the dynamic response of negative photoconductivity at 304 K was investigated, when the AgNWB was irradiated (on/off circles) by 142.3 mW continuous laser under a bias voltage of 0.1 V. A significant decrease in current of the AgNWB was observed and the response speed was much faster than that of the subsequent process with further current decrease. When the light was turned on, the current immediately decreased from 3.216 mA to 3.190 mA, and then the current slowly decreased from 3.190 mA to 3.182 mA. When the light was turned off, the current immediately increased from 3.182 mA to 3.208 mA, and then the current slowly increased from 3.208 mA to 3.211 mA. During every period of on/off circle, the fast response process was related mainly to the photo-induced SPP-electron interactions, while the slow response process was related to the SPP-induced heating. Both SPP-electron scattering and SPP-induced heating had contributions in photoconductivity. SPP-electron scattering plays the dominate role for short characteristic time scale, i.e., for pulsed light having a small pulse duty factor.
To avoid possible influence on negative photoconductivity of the AgNWB caused by the Joule heating and the SPP-induced heating processes, we studied the conductive properties of the AgNWB under small bias voltage of 0.1 V and pulsed laser field with small pulse duty factor. Thus the dynamic responses of negative photoconductivity for the AgNWB illuminated by pulsed light were studied in the temperature range 304 K to 77 K. As an example, Fig. 2(d) and Fig. 2(e) show the dynamic responses at 304 K and 77 K, respectively. Figure 2(e) clearly shows that the dark current Id maintains a constant 4.542 mA at 77 K, while it is 3.216 mA with small fluctuation at 304 K during 5 seconds as seen in Fig. 2(d). When the light was turned on, a significant decrease in current of the AgNWB was observed too. By comparing Figs. 2(c), 2(d) and 2(e), one sees that the fast process of SPP-electron scattering gives most contribution to current change in the case with short characteristic time scale. The current change |ΔI| due to the illumination as a function of temperature is shown in Fig. 2(f) (pink line). Based on the above experimental results, we may have a linear relationship between the |ΔI|-1/4 and temperature [see Fig. 2(f), green line]. Furthermore from all the data shown in Fig. 2, one sees that during temperature increasing, the dark resistivity ρd of the AgNWB increases and the photo-induced resistivity change Δρ decreases. This is due to the particular natures of the SPP-electron interaction and phonon-electron interaction as will be discussed in more details later. The ratio of resistivity change has been plotted as a function of temperature in Fig. 3(a) . The photo-induceddecreases as the temperature increases from 77 K to 304 K, and there is a nonlinear relationship between theand temperature. The monotonically linear increases of the current change |ΔI| with laser power were observed both at 77 K and 304 K as shown in [Fig. 3(b)]. The results indicate that the SPP-electron interactions are more sensitive to laser power at lower temperature. This transmission behavior of the AgNWB can be ascribed to the SPP-electron interaction on the surface of the AgNWs. We also observed a weak dependence on the angle of excitation, which is due to the fact that the nanowires in the macrobundle are not strictly parallel to each other, as seen in SEM image Fig. 1(b).
4. The theoretical analyses
We propose a model to explain the experimental observations. From Drude theory and Matthiessen's rule, the resistivity is where is the dark resistivity in the absence of irradiation and is the correction due to the interaction between electrons and SPPs excited by external light. is the bulk resistivity. The bulk resistivity due to the scattering between electrons and impurities and phonons has a linear relation with temperature. ρ 2 is the resistivity due to surface scattering. , the bulk mean free path, Fermi velocity and D the radius of the nanowire. ρ 2 is temperature independent due to the cancellation of the temperature dependence of ρ 1 and l 1. Overall we have where is the relaxation time of an electron in metal nanowires in the absence of irradiation, which includes the effects from scattering of electrons with impurities, phonons and surfaces.
In the presence of irradiation (with frequency wavelength 532 nm), additional scattering channel is generated due to the interaction between electrons and excited SPPs. The external laser field excites SPPs due to the surface roughness as seen in Fig. 1(c) . The resistivity due to the interaction between electrons and SPPs has the form20]. Thus we have where is the laser power. For lower temperature with longer relaxations time (longer mean free path), there is bigger probability for electrons scattered by SPPs before it scatters with other degree of freedoms (phonons for instance). Therefore we obtain and the formula for current
The accurate value of γ depends on the electronic structure, the surface roughness, the SPP distribution, etc. The calculation ofγ is beyond the scope of this paper and will be discussed in the future. Yet, our theoretical analysis reveals the basic physical picture and leads to a few important results. First, SPP-electron interaction leads to another scattering channel and results in the decrease of current. As temperature increases, increases and decreases as observed in our experiments. Second, the SPP-induced resistance has weak dependence on the laser frequency and vanishes when the radius of nanowire D is very large, which are consistent with our experimental observations (not shown in this paper). Third, our theory shows that (since). Figure 2(f) shows a nice linear relation between and T. Furthermore, from the fitting of the curve (green line) in Fig. 2(f), we have which agrees very well with the value from dark resistance as obtained from Fig. 2(b). Fourth, as also seen in Fig. 3(b). Moreover our theory points out that
From the slop for versus P in Fig. 3(b), we have l0(T = 77 K) / l0(T = 304 K) = 1.49, which is consistent with l0(T = 77 K) / l0(T = 304 K) = ρd (T = 304 K) / ρd (T = 77 K) = 1.41 from Fig. 2(b). Our theory
5. Discussion and conclusion
Finally, we would like to point out the essential roles of SPPs. First, we emphasize the importance of SPP-electron interaction, rather than the direct interaction between photons and electrons in our systems, though the SPPs are excited by the photons. The states near Fermi energy play the dominate roles in transport. Thus the photon [with high energy (a few eV >> 300 K)] induced inter-band transition makes less contribution to transport processes. While photons are not directly involved into electronic transport, they excite SPPs. The low energy SPPs (due to the dispersion relation in low dimension) induce intra-band transitions and make measurable contribution to the transport. Second, the effective excitation of SPPs depends on the roughness of surface . To support this point, we deposited a strip of Ag thin film with thickness of 50 ~60 nm and width of 1 mm onto an optical flat surface of an insulating substrate by vacuum thermal evaporation [see Fig. 4(a) ]. The two ends of the strip were connected with conductive silver paint electrodes. When the current for the Ag film was measured under similar condition, we found no change with turning on irradiation [see Fig. 4(c)]. But after adding some ultrasonically dispersed Ag wires [see Fig. 1(c)] on the Ag film surface under the laser spot [see Fig. 4(b), thus increasing the surface roughness], we found measurable change (decrease) of current with irradiation [see Fig. 4(d)]. These facts point out the importance of surface roughness, indicating the essential roles of SPPs. Third, the absence of measurable current change for a flat Ag film with irradiation further indicates that the negative conductivity we observed in the AgNWB is a surface effect, instead of bulk effect. Consequently, the negative conductivity is unlikely related to thermal effect. And the time interval of the laser pulse is short (due to the optical chopper), leading to negligible effect. Also usually the resistance due to thermal effect (phonon scattering) has different temperature dependence from that of the resistance change () we observed in experiments. Fourth, the photo-induced voltage has been avoided by putting the laser spot near the geometric center of the AgNWB .
In summary, considerable negative photoconductivity has been observed experimentally and demonstrated for macroscopic length AgNWB. The theoretical investigation clearly shows that the photo-induced SPPs have a notable influence on transportation of electrons in AgNWB. SPP-electron interaction leads to another scattering channel and the decrease of current. We expect that the photoconductive behavior will be more pronounced if AgNWs have smaller dimensions and higher surface-to-volume ratio. Our experimental and theoretical studies have shed new light on the novel phenomena from combination of photonic and electronic properties of AgNWBs. Their unusual photoelectrical properties will be useful for scientific research and technical applications, such as a high sensitivity photoconductor.
This work was supported by the National Natural Science Foundations of China (No. 10711120167, 10744004, 10874020, 10774085 and 10974108), MOST Program (No. 2006CB0L0601) of China and a grant of the China Academy of Engineering and Physics.
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