**3.3. TEM characterization**

At pH = 5.0 Figure 6b, isolated nanowires were fragmented also with small aspect ratio. Porosity within the individual segments could not be confirmed, however, we suspect that either the nanowries themselves are brittle and fragment post-isolation, or that the deposition results in porosity which leads to fragmentation, once the structural support of the template is removed. It is unclear at this time what the mechanism for the resulting

In contrast to both previous samples, high-aspect ratio nanowires were successfully synthe‐ sized using a plating solution with pH = 3.1, Figure 6c. Image analysis showed that aspect ratios from 20:1 to more than 50:1 were achievable using this approach. More work is needed to better understand what mechanisms are responsible for controlling the transport mecha‐ nisms taking place in the nano-channels. From these data, we determined pH = 3.1 would be

Co-deposition of binary alloys and multilayer nanowires has been reported elsewhere [29-31]. Here, platinum (Pt) and iridium (Ir) metal atoms are deposited through the AAO channels by

By cycling the potential in a range below the equilibrium potentials for both ion complexes, the kinetics of deposition can be modulated to ensure that both elements are deposited in

For this study, the potential range that produces a desirable Pt:Ir composition was unknown. We therefore selected five different potential ranges, Table 1, and prepared nanowires using each cycling range. All five potential ranges spanned 250 mV of potential but used different

> **Potential range (V) vs. Ag/AgCl Average Pt:Ir ratio (%)** ∆U = 0.20 to -0.15 68:32 % ∆U = 0.02 to -0.15 67:33 % ∆U = 0.00 to -0.15 62:38 % ∆U = 0.00 to -0.20 68:32 % ∆U =-0.05 to -0.15 85:15 %

Following deposition, nanowires were isolated and composition was tested using energy dispersive spectroscopy (EDS). Results in Table 1 show compositional fractions ranging from 62:38% to 85:15% platinum were attained by varying chemistry. While the exact mechanisms responsible for the differences in concentration are not well understood at this stage, we do know that shifting the potential range used for deposition affects the deposition kinetics inside

4- in solution.

capable of producing nanowires with preferred morphological structure.

electrochemically reducing from platinum ions [PtCl₆]⁴⁻ and iridium ions [IrCl6]

starting potentials (Uo) from Uo = -0.5V to Uo = 0.2V vs. Ag/AgCl.

**Table 1.** Deposition potential ranges and resulting PtIr nanowire fractional composition

**3.2. Controlling Pt: Ir composition with potential**

216 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

microstructure may be.

desirable quantities.

the nano-channels.

TEM analysis was performed on 60:40% Pt:Ir nanowires to further characterize material properties. Figures 7a and 7b are bright-field and dark-field TEM images of platinum nano‐ wires, respectively, deposited at the optimal conditions for creating hermetic AAO-feed‐ through assemblies.

Figure 7a shows a gross image of a single isolated nanowire. Due to thickness and low magnification, no structural information (grain size, orientation, etc.) is distinguishable. At higher magnification and in dark field mode, however, grain boundaries and morphology can be distinguished. In Figure 7b, we estimate average grain size in the range of 5-10 nm, and they show no preferential growth orientations. There is some contrast observed between grains along the perimeter of the wire versus grains occurring within the central axis of the nanowries. This may suggest that contact with the AAO nanochannel surfaces may direct grain growth in some preferred orientations, however more studies are needed.

**Figure 7.** Bright-field (a) and dark-field (b) images of platinum nanowires.

TEM diffraction patterns of the deposited nanowires, Figure 8, showed concentric ring patterns, confirming the dark field observations that the nanowires were deposited with a polycrystalline structure, with no preferred orientation, and with an average grain size of 3-5 nm. Grain sizes calculated from x-ray diffraction patterns using Scherrer formula may underestimate grain size, as strain effects can impact patterns. The radius(r) of the diffraction rings varies with h, k, l as shown in equation (1):

$$r \propto \sqrt{h^2 + k^2 + l^2} \tag{1}$$

where h, k and l are the Miller indices that represent the crystallographic plane. The results of the calculations using equation (1) showed that the rings correspond to the planes (111), (200), (220) and (311) which represent a typical face-centered cubic (FCC) structure (Figure 10).

**Figure 8.** Electron beam diffraction pattern of platinum-iridium nanowire with fcc structure.

A HRTEM image of an individual platinum-iridium nanowire is shown in Figure 9. Since the values of the lattice parameters of platinum and iridium are too close to each other, the (−1−1 1), (1 1 1) and (0 0 2) planes labeled in Figure 11 represent the FCC crystal structure that may belong to either platinum or iridium (JCPDS 04-0802) or an alloy of the two. Consequently, the twin at the grain boundary might be due to the effect of a platinum or iridium alloying grain, indicating a bimetallic particle [33].

**Figure 9.** HRTEM image of an individual platinum-iridium nanowire

### **3.4. Conductivity measurements**

TEM diffraction patterns of the deposited nanowires, Figure 8, showed concentric ring patterns, confirming the dark field observations that the nanowires were deposited with a polycrystalline structure, with no preferred orientation, and with an average grain size of 3-5 nm. Grain sizes calculated from x-ray diffraction patterns using Scherrer formula may underestimate grain size, as strain effects can impact patterns. The radius(r) of the diffraction

2 22 *r hkl*

where h, k and l are the Miller indices that represent the crystallographic plane. The results of the calculations using equation (1) showed that the rings correspond to the planes (111), (200), (220) and (311) which represent a typical face-centered cubic (FCC) structure (Figure 10).

+ + (1)

a

**Figure 8.** Electron beam diffraction pattern of platinum-iridium nanowire with fcc structure.

indicating a bimetallic particle [33].

A HRTEM image of an individual platinum-iridium nanowire is shown in Figure 9. Since the values of the lattice parameters of platinum and iridium are too close to each other, the (−1−1 1), (1 1 1) and (0 0 2) planes labeled in Figure 11 represent the FCC crystal structure that may belong to either platinum or iridium (JCPDS 04-0802) or an alloy of the two. Consequently, the twin at the grain boundary might be due to the effect of a platinum or iridium alloying grain,

rings varies with h, k, l as shown in equation (1):

218 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

Single nanowire, conductivity measurements were also performed on 60:40 Pt-Ir nanowires and compared against platinum nanowires prepared by a similar method. Figure 10a and 10b show scanning electron micrographs of lithographically fixated platinum and platinumiridium nanowires, respectively, fixed between two lithographically patterned Au contacts. Platinum nanowires were used as a comparator as this is the only other known method of synthesizing similar nanoscale feed-through constructs. The method for their synthesis is described elsewhere [24]. Resistivity measurements were taken across the nanowire bridge and lead resistivity was subtracted out based on the bridging nanowire's location on the source and sink contact strip (approximately 2 µm for both samples).

Three representative current-voltage plots for both nanowire species are plotted in Figure 11. The slopes of these plots are inversely related to resistance [I = (1/R)V], therefore smaller slope magnitude suggests reduced resistance. For the six nanowires tested here we can calculated almost 2-fold improvement in conductivity for the Pt-Ir nanowires vs. pure platinum, which is consistent with known intrinsic properties for both metals (*ρPt* = 105 nΩ m and *ρIr* = 47.1 nΩ m, respectively). More investigations are needed in this space.

**Figure 10.** SEM micrograph of the testing device used for electrical conductivity measurement of a) single platinum nanowire and b) single platinum-iridium nanowire.

vity measureme

nowires.

**Figure 11.** Current vs. voltage plots demonstrating the improved conductivity of platinum-iridium nanowires.

### **3.5. Helium leak testing**

Figur

**3.4.**

**Effect of d**

mage of an indiv

vidual platinum

m‐iridium nanow

wire

**hemical com**

**mposition**

**n of the nan**

**nowires**

tio (%)

d out by app e fabricated de deposited on rodes was 2 μm

e platinum nan

h of the platin es to that of

mposition of t on the atomic at small chan ely large chan 5 V changed t

the nanowires c ratio of plati ges in the ap nges in the am the Pt:Ir ratio

s was num‐ pplied mount from

plying an elec evices for plat n top of disp

ctrical tinum ersed

single

num‐ vealed

nowire and b)

num and plati platinum rev

m.

chemical com tential ranges 1 suggest tha caused relativ om 0 to ‐0.05

erage Pt:Ir rat

s were carried ographs of the were sputter‐ nd drain electr

ent of a) single

ealed that the of different pot ults of Table ntial sweeps c sitive limit fr

ntial ranges.

um nanowires electron micro m Au stripes w allel source an

Ave 68:3 67:3 62:3 68:3 85:1

32 % 33 % 38 % 32 % 15 %

anowires reve ws the effect o wires. The resu on of the poten nge of the po

nt applied poten

latinum‐iridiu the scanning e wo thin‐ film n the two para

trical conductiv

m‐iridium nan

**l on the ch**

um‐iridium na l. Table 1 show idium nanow gative directio ample, a chan

g/AgCl

wires in differen

tinum and pl and b show ely, where tw he gap between

used for elect

igure 13 show ge currents m or the platinum

**n potential**

esized platinu ition potential d platinum‐iri sitive and neg wires. For ex

ange (V) vs. Ag

the Pt‐Ir nanow

**urements**

ts of the plat es. Figure 12 a res, respectiv distance of th

testing device

age plots in fi of the averag onductivity fo

**deposition**

of the synthe on the deposi ectrodeposited n both the pos nt of the nano Table 1).

Potential ra 0.2 to ‐0.15 0.02 to ‐0.15 0 to ‐0.15 0 to ‐0.2 ‐0.05 to ‐0.1 composition of

55

**vity measu**

measuremen ngle nanowire dium nanowir tact pads. The

rograph of the anowire.

e current‐volt es. The ratio o imes higher co

EDS analysis hly dependent um of the ele ntial ranges in idium conten 8% to 85:15%(T

The high iridiu poten of ir 62:38

Table

e 1. Chemical

**Conducti**

conductivity ntial across sin platinum‐irid owires as cont

re 12. SEM micr num‐iridium na

representative um nanowire roximately 2 ti

**3.5.**

The poten and nano

Figur platin

The iridiu appr

a

1 µm

nanowire and b) single platinum-iridium nanowire.

**Figure 10.** SEM micrograph of the testing device used for electrical conductivity measurement of a) single platinum

1 µm

220 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

Pt-Ir nanowire in template assemblies were prepared with 60:40 (Pt:Ir) composition, and subjected to helium leak testing. Figure 12 shows a cross-sectional SEM micrograph of a fractured AAO template following platinum-iridium nanowire deposition. The conductive metallic nanowires are clearly seen as white, high-aspect ratio elements surrounded by the nano-channeled AAO (darker surrounding material). The right side of the image shows the 20 nm aperture side of the template and some of the bifurcations and branching can be resolved. The left side of the image shows the 200 nm aperture side of the template. Here it can be seen that complete filling of the channels has not been accomplished in this sample.

Helium leak testing results for these samples were taken an averaged 1.5 x 10-11 mbar L sec-1 (variance not calculated). These results suggest that these structures would meet industry standards for hermetic leak rates [27, 28], which have been reported at values as high as 2 x 10-10 mbar L sec-1. For comparison, samples prepared using other potential ranges were also tested and found to be one to two orders of magnitude more leaky. These results suggest that it may be possible to fabricate hermetic feed-throughs using these types of nano-scaled constructs, however care must be taken to ensure that the metal deposition procedure produces non-porous, high-aspect ratio conducting elements.

**Figure 12.** SEM micrograph of the cross section of platinum-iridium nanowires grown in AAO pores.
