C-shape

[Jan Dominquez]

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We introduce a method based on directed molecular self-assembly to manufacture and electrically characterise C-shape gold nanowires which clearly deviate from typical linear shape due to the design of the template guiding the assembly. To this end, gold nanoparticles are arranged in the desired shape on a DNA-origami template and enhanced to form a continuous wire through electroless deposition. C-shape nanowires with a size below 150nm on a \(\hbox SiO_2/\hbox Si\) substrate are contacted with gold electrodes by means of electron beam lithography. Charge transport measurements of the nanowires show hopping, thermionic and tunneling transports at different temperatures in the 4.2K to 293K range. The different transport mechanisms indicate that the C-shape nanowires consist of metallic segments which are weakly coupled along the wires.

The degree of miniaturization achievable through DNA origami guided assembly of gold nanowires, however, is still to be determined. Here we explore the feasibility of the DNA origami technique for creating nanodevices within the current size limits by fabricating a split-ring resonator (SRR), a tiny LC circuit24. The main application of this design is the generation of a metamaterial25. The inductor in a SRR is brought about by an incident electromagnetic wave which produces a time dependent electromotive force. The topology of the SRR corresponds to two C-shape concentric wires with a slit (capacitor) on opposite sides having radii \(r_\mathrmext\) and \(r_\mathrmint\), with \(r_\mathrmext > r_\mathrmint\), and a gap d between the perimeter of the wires26. As a proof of concept of this device, a C-shape Au nanowire is used as a case study to achieve one of the rings of the SRR. The electrical behavior of this structure will be studied as a function of temperature. The outcome of this experiment is essential for the further construction of the metamaterial, because the electrical properties of the metallic structures determine the optical properties later achievable in ordered arrays of these nanostructures. These tests need to be performed in order to select suitable candidates for the construction of the metamaterial.

The electrical characterization of nanowires was done using electron beam lithography (EBL)19,22,23,27,28,29 with electrodes fabricated to the ends of the C-shape wires12,20,30,31,32. Electric charge transport of such wires shows a widespread range in resistances from a few tens of ohms up to several gigaohms. These results suggest that the fashioning of metal NPs on the origami nanostructure and their coalescence during metallization control the resistance, whereas the length and width of the final structure hardly influence the electrical properties19,27,28,33. Temperature-dependent electrical characterization of nanowires generated by DNA metallization has shown that these wires are not completely continuous33. Merely, they often contain gaps between the metal nanoparticles which appear during growing and merging the AuNPs on the surface of the DNA origami34

Here, we analyze the electric transport of C-shape gold nanowires based on small DNA origami templates (\(90\hbox nm \times 70\hbox nm\) DNA origami nanosheets) having three functionalized sides, holding eight gold nanoparticles conforming one of the rings of a SRR (see Fig. 1). The DNA origami nanosheet design is shown in figure S1 and the staple strand sequences are given in tables S1 and S2 in the supporting information (SI). Electroless gold growth is applied to selectively grow the gold nanoparticles until they merge into nanowires. Finally, this work demonstrates the fabrication of single and isolated conductive nanowires with a C-shape. They are precisely contacted by metal using EBL to understand the charge transport characteristics at different temperatures. In particular, we need to answer the following questions:

Specifically, the C-shape was designed for generating a metamaterial for optical applications. For such applications the electrical conductivity is important, because it determines the ability to couple electromagnetic waves. Optical characterization of the metamaterials is not possible before ordered deposition of the origami structures can be achieved. It is, however, necessary to characterize the nanostructures before the development of the deposition, because the deposition depends on the properties of the nanostructures, as well. Therefore, the first step in the construction of a metamaterial is the testing of the electrical properties. In order to proceed with the metamaterial production, it is not only essential to have single structures with good properties, but these structures are also needed with good statistics. We therefore investigate the statistical properties of the binding of the nanoparticles and the distribution of their electronic properties. To the best of our knowledge, no such study has been performed up to date.

where n, m and p are the number of available binding sites, number of AuNPs attached per origami nanosheet, and average probability of AuNPs site occupation, respectively. These values were obtained from the histograms of the number of attached AuNPs per nanosheet, which have been compiled from the AFM data, to accomplish the binomial distribution for each experimental condition. The results of the statistical analysis are given in Table S3 in the SI. The solid lines in the histograms in Fig. 2e and SI-figure S4 show the calculated binomial distribution following equation (2). It can be clearly seen that for a high attachment probability, the experimentally achieved distribution deviates from the binomial distribution. This is an indication that at these high attachment probabilities steric effects and/or Coulomb interactions between the nanoparticles may play an important role in determining the yield of successful binding events.

In particular, the average attachment probability was calculated to be 0.80 for \(T_ \rm inc\) = 30 min, a concentration of 20 mM \(\mathrm MgCl_2\) and 2.5:1 molar ratio of functionalized AuNPs to DNA origami, as seen from Table S3. The highest AuNPs site occupation probability reported in literature is \(p=0.99\) for a DNA origami nanorail (\(6\times 2\) helix bundle) with base-pairing interaction for four capture strands per binding site35. The reduction of the attachment probability is most likely caused by the geometry of the DNA origami and, related to this, by the number of available capture strands per particle. The geometry, however, is crucial for the application of the origami technique in nanoelectronics. Therefore, the attachment probability of 80% is used for the electrical characterization of these proof-of-concept devices discussed here.

Resistance versus temperature curves for two Au-metallized C-shape nanowires, named NW-1 and NW-2, templated by DNA origami in the 4.2 K to RT temperature range. Scanning electron microscopy images of the two nanowires can be seen as inset figures.

The high resistance values are mainly caused by imperfections in the metallization of the wires, which cause gaps between the nanoparticles even after growth. These gaps can be clearly seen in the high magnification SEM images in Fig. 4.

(a) Representative diagram of hopping, thermionic and tunneling injection processes over the barrier between metallic contacts and nanoparticles. (b) Inverse temperature dependence of conductance for NW-1 and NW-2. (c) Linear fit of data shown in (b) in the high-temperature range, in which hopping is the dominant charge transport mechanism. (d) Plot of \(ln(I)/T^2\) according to the Richardson-Schottky model describing thermionic emission. (e) Linear fit of the data shown in d) in the intermediate temperature range, in which thermionic emission dominates charge transport.

where \(\Phi\) is the barrier height, \(A^*\) is the effective Richardson constant, \(\epsilon\) and d are the permittivity of vacuum and the barrier width, respectively. Figure 5d shows graphs of \(\ln \left( \fracIT^2\right)\) as a function of \(T^-1\) for both wires in the temperature range from 4.2 K to RT. For both wires, this plot shows a linear region, in which thermionic emission is most likely dominating. The temperature range in which this holds true depends on the morphology of the wire. In our measurements, it extends from 15 K to 150 K and from 30 K to 250 K for NW-1 and NW-2, respectively, as shown in Fig. 5e.

Subsequently, AuNPs were enhanced to homogeneous C-shape metallic nanowires. The growth of the AuNPs was controlled by means of 1:1 and 1:2 mixtures of gold plating solution and 20 mM \(\mathrm MgCl_2\) in a 1 \(\times\) TAE buffer. These mixtures were dropped onto the AuNP:DNA origami conjugates with different incubation times, 2, 7, 14 and 20 min. Rectangular DNA origami structures on \(\mathrm SiO_2/Si\) substrates and AuNP assemblies were characterized by AFM operating in tapping mode using a Bruker MultiMode 8 Scanning Probe Microscope and aluminum reflex coated tips (Tap150Al-G from Nanoandmore, force constant \(5 \hbox Nm^-1\), tip radius \(

In order to contact individual C-shape nanowires for electrical transport measurements, sets of alignment marks in parallel arrays with \(8\,\upmu \hbox M\) gaps in between were patterned by means of electron beam lithography on \(\mathrm SiO_2/Si\) surface, as seen in Fig. 3a, using experimental conditions as previously reported19. The relevant positions of the individual wires were determined relative to the alignment marks using SEM. These locations were used to obtain corresponding e-beam exposure positions in order to place electrical contacts on the nanowires. SEM images confirmed that the DNA origami templated C-shape wires are between electrical contacts with high accuracy.

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