Electrical and optical effects in molecular nanoscopic-sized building blocks

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Physica E 19 (2003) 126 – 132 www.elsevier.com/locate/physe

Electrical and optical e
ects in molecular nanoscopic-sized building blocks W.P. Kirka;∗ , K.L. Woutersb , N.A. Basita , F.M. MacDonnellb , M. Taoa , K.P. Clarka a Department

of Electrical Engineering and NanoFAB Center, University of Texas, P.O. Box 19072, Arlington, TX 76019, USA of Chemistry, University of Texas at Arlington, P.O. Box 19072, Arlington, TX 76019, USA

b Department

Abstract A biomimetic strategy is used to bridge the gap from the molecular scale to the microscale. Dendritic, conformationally rigid ruthenium(II)polypyridyl complexes have been synthesized to form a set of nanometer-sized building blocks (NBBs) (1.6 –5:0 nm) with discrete ‘tertiary’ structures. We postulate that thin lms constructed from NBBs with di
ering tertiary structures will form ordered arrays with di
ering ‘quaternary’ structures and, presumably, di
ering electronic properties. Experimental studies show that thin lms of these molecules are conductive, display electric eld-modulated conductivity, and enhanced conductivity upon visible irradiation. Signicantly, changes in the molecule’s overall shape (tertiary structure) have a measurable e
ect on the electrical properties of the lms prepared from them. Importantly, these NBBs are chemically robust and structurally tunable. We intend to exploit these properties as well as their newly discovered electronic, optical, and, potentially even, chiro-optical properties so as to provide a new, added dimension in molecular electronics and chiral optical devices. ? 2003 Elsevier B.V. All rights reserved. PACS: 61:46:+w; 73.63.Bd; 78.67.Bf Keywords: Nanoscopic building blocks; Molecular electronics; Chiral optics; Ruthenium polypyridyl; Biomimetic

1. Introduction Molecular electronics derives inspiration from early work by Aviram and Ratner [1], who espoused the idea that organic molecules could function as p–n junction diodes. Over the past few years, a urry of research has appeared on the development of electronic devices from nanometer-sized chemical com-

∗ Corresponding author. Tel.: +1-817-272-5632; fax: +1-8177458. E-mail address: [email protected] (W.P. Kirk).

ponents such as carbon nanotubes, buckyballs, and nanowires. Two recent articles [2,3] report signicant progress in demonstrating simple circuits in semiconductor nanowires and carbon nanotubes. A group at Harvard used p-type silicon and n-type gallium nitride nanowires to form an array of p–n junctions [2]. They assembled OR and AND logic circuits with their junction arrays. More interestingly, their nanowire circuits were assembled without ‘top-down’ methods such as lithography, except for the electrical contacts. Instead, the nanowire circuits were self-assembled by microuidics. In the Netherlands, a group demonstrated logic circuits with up to 3 carbon nanotube

1386-9477/03/$ - see front matter ? 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1386-9477(03)00298-4

W.P. Kirk et al. / Physica E 19 (2003) 126 – 132

properties, including eld e
ect and spin-correlation between the electrons in the electrode and molecule. Others have established that simple single-organic molecule (or a-few-molecule) devices can function as electronic components, including diodes [13], switches [9], and memories [10]. The bottleneck in demonstrating a molecular circuit with a discrete number of molecules is the interconnection technology, i.e. a practical method or methods to connect a large number of molecular devices into a functional circuit. Many of the 2-terminal molecular devices demonstrated so far have been characterized in the so-called ‘nanopore’ structure [13], which is dicult to scale up for the construction of a molecular circuit. Lithographic techniques may ultimately reach the atomic scale; but using today’s practical technologies, another solution to the interconnect problem would be to build larger nanoscopically sized molecules, thus requiring fewer of them to bridge contacts. In this manner, molecularly programmed self-assembly

transistors [3], including inverter, logic NOR, static random access memory, and ring oscillator. However, the fabrication involved a time-consuming and arduous process to select the right carbon nanotubes on which transistors were fabricated. Finally, a Berkeley group has demonstrated a single-electron transistor using a CdSe nanocrystal [4,5]. The use of organic molecules as the active nanoelectronic component is another critical development, which is a focus of the reported work. Compared to nanoelectronics based on semiconductor nanowires [2,6] or carbon nanotubes [3,7], organic molecules [8–10] are the smallest nanoelectronic devices under investigation and o
er a tantalizing variety of potential components and properties. For example, two teams, one at Berkeley/Harvard [11] and the other at Cornell [12], showed that single molecules of metal-organic complexes demonstrate a eld e
ect and that the metal ion’s redox state and paramagnetism have considerable inuence on the electronic

2+

127

NN N Ru N NN

NN Ru NN

N N

Λ

8+

N N N Ru N N N

2+

N N

1° to 2°

∆

diameter 3.2 nm

N N N Ru N N N

N N

N N N N Ru N N N N

Ru4

N N Ru NN N N

2° to 3° 12+ N N N Ru N NN

N N N Ru N N

N NN

N N

N N

N N

N NN N Ru N N

N N

N

N N Ru NN N N

12+ NN

N N Ru N N N N

diameter 4.3 nm

N

T-Ru6

2° to 3° NN

N N N N N Ru N N N

N N N Ru N N N

N N N Ru N N N

N N Ru N N N N

Ru N N

NN N N

N NN N Ru N N

N N

NN N Ru NN N

P-Ru6

Fig. 1. Structure and mirror image relationship of Ru(II) trisphenathroline chiral building blocks; their assembly into rigid tetramers (Ru 4 ); and a conceptual schematic as to how such tetramers can be linked to form hexamers (Ru 6 ) with di
ering tertiary structures.

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processes as well as tailored molecular electronic and/or optoelectronic properties could be used to rapidly and cheaply build devices in a large-scale, low-cost industrial process. Commercial Si integrated circuits run at several GHz [14], and high-speed semiconductor devices have shown cuto
frequencies, a gure of merit for device speed, up to 500 GHz [15,16]. While molecular devices are inherently slower, by orders of magnitude, than carrier transport in a semiconductor [17,18], molecular assemblies/electronics almost certainly will possess unique electronic and optoelectronic properties that are dicult to come by in semiconductor nanoelectronics. In fact, two recent developments in photovoltaics both involve organic molecules or hybrids as the absorption medium. A Berkeley group demonstrated a hybrid solar cell with CdSe nanorods and poly-3(hexylthiophene) molecules as the absorption medium with a power conversion eciency of 1.7% under Air Mass 1.5 [19]. A Swiss group developed a dye-sensitized TiO2 nanoparticle solar cell with an eciency of 10% under AM1.0 [20]. We are examining the electronic and optoelectronic properties of an interesting class of macromolecules based on the well-known ruthenium(II)tris(bipyridine) cation (Rubpy). Rubpy and its derivatives have been extensively used in applications such as redox mediators [21], solar sensitizing dyes [20,22] and electroluminescent dyes [23] because the Rubpy complex is both chemically robust and has favorable redox, optical, and luminescent properties. For example, Ru(II) polypyridine molecules, undergo readily reversible ligand or metal-based redox reactions at potentials of about −1:0 and +1:3 V respectively (vs. SCE reference electrode) [21]. They typically possess a strong absorption band (
max ∼ 480 nm) which is associated with a long lived (∼ 100 ns), excited-state in which the Ru2+ is oxidized to Ru3+ and the polypyridine ligand is reduced by 1 electron; this charge separated state has been exploited for numerous light-to-energy or energy-to-light schemes [24]. The macromolecules used herein for molecular electronics are composed of multiple Rubpy units linked in a rigid manner to provide multinanometer-sized assemblies of luminophores which possess a well-dened, and chemically tunable, overall conguration [25,26]. Furthermore, the Rubpy complex is chiral and we

exploit this property by working with enantiopure complexes to control the global or ‘tertiary’ structure (shape) of the macromolecule as well as incorporating useful chiro-optical properties. We describe herein some of the unusual electronic and optoelectronic properties of thin lms of these molecules when contacted with lithographed microcontact arrays.

2. Experimental procedure and results This research reports on the electronic and optoelectronic properties of Ru polypyridine macromolecules as shown in Fig. 1. These molecules serve as the nanoscopic building blocks (NBBs) to assemble 2-terminal molecular structures and simple logic molecular circuits on a solid surface.

Au electrode

NBB

SiO 2

+

n Si substrate VSD VG

Fig. 2. (Top) Micrograph of gold contact pad on silicon. (Bottom) Schematic side view of substrate-electrode structure used to contact NBBs. Spacing between Au electrodes was 1
m. Oxide thickness was 20 nm. VSD is source–drain voltage source. VG is gate potential.

W.P. Kirk et al. / Physica E 19 (2003) 126 – 132

129

-7

1.0x10

-8

Filtered drop, T, Ru6, Cl, glass, hot plate baked at 100C

8.0x10

-8

6.0x10

L

-8

N

N

N

N

N

N

hole

4.0x10

- e-

+ e-

Current (A)

-8

2.0x10

0.0

L

-

N

N

N

N

N

N

Ru2+ + e-

-8

-2.0x10

ion

-8

-4.0x10

- e-

Ru3+

-8

-6.0x10

electron

-8

-8.0x10

-7

-1.0x10

-6

-4

-2

0

2

4

6

Voltage (volts) Fig. 3. Current vs. voltage characteristics of T-Ru6 molecules on glass slides at repeated time scans. Samples were baked at 100◦ C for 2 h. Electron conduction through ligands is indicated at negative voltages, ion conduction is indicated at low voltages, and hole-hopping conduction is indicated at positive voltages.

-12

I (A)

150x10

100

50

0 0

2

4

6

8

VSD (V) Fig. 4. Di
erence in conductivity of two lms of the hexamer (Ru6 ). Open circles denote the ‘twisted’ (P-Ru6 ) isomer. Closed circles denote the ‘at’ (T-Ru 6 ) isomer.

Thin lms of these NBBs were prepared either by applying a drop of solution and letting it evaporate (from MeCN (as the PF− 6 salt) or from water (as the Cl− salt) or by spin-casting it from MeCN solution.

The microcontact pattern used is shown in Fig. 2, top. This pad was patterned in gold on glass and on a silicon wafer with a top SiO2 layer. A side-view schematic of the device is shown in Fig. 2 (bottom), which illustrates how the measurements were made and voltages applied. At a microcontact separation of 1:0
m, thin lms of the hexamer T-Ru6 on a glass substrate show electrical conductivity at voltages less than −1:8 V and greater than +2:0 V which increases as a function of the applied voltage. We postulate that the mechanism of conduction at negative potentials occurs by reduction of the coordinated phenanthroline and tpphz bridging ligands and transport through the -conjugated network. At positive potentials, conduction is likely occurring via oxidation of some of the Ru2+ sites to Ru3+ , thus providing a hole-hopping pathway through the lm. Redox processes at +1:35 V(Ru2+=3+ ), −0:78 V (tpphz=tpphz− ) and −1:36 to −1:54 V (phen=phen− ) relative to SCE are observed in single molecules of T-Ru6 as determined by solution cyclic voltammetry [27,28], which

W.P. Kirk et al. / Physica E 19 (2003) 126 – 132

130 5.00E-009 4.00E-009 3.00E-009 2.00E-009

IDS (A)

1.00E-009 0.00E+000 -1.00E-009 Vg = -0.0 V Vg = -1.0 V Vg = -2.0 V Vg = -3.0 V Vg = -4.0 V Vg = -5.0 V Vg = -6.0 V

-2.00E-009 -3.00E-009 V onset

-4.00E-009 -5.00E-009 -6

-4

-2

0

VDS (V) Fig. 5. Source–drain current vs. source–drain voltage at negative gate voltages in 1 V intervals. A typical onset voltage is shown.

-1.5 Potential

-2.0

- 6.0 V - 4.5 V 0V

LUMO

-2.5 SiO 2 SiO

(conduction)

V onset

VG

-3.0 HOMO (valence)

-3.5

-4.0

-4.5 -6

-5

-4

-3

-2

-1

0

VG Fig. 6. Onset voltage of ligand electron conduction as a function of gate voltage. Smooth curve is a second order polynomial t. Inset shows a schematic illustration of the potential diagram for the relation between gate potential and molecular orbital state levels.

W.P. Kirk et al. / Physica E 19 (2003) 126 – 132 -9

110x10

(A)

100

IDS

corresponds to the onset of conduction potentials seen in Fig. 3. While the absolute reference potentials for the solution and thin lm have not been determined at this time, it is clear that the redox inactive window of 2:14 V for the solution study (−0:78 to 1:36 V) corresponds well with the redox inactive window (excluding the ion capacitance, vide infra) of 3 V for the thin lm study (−1:0 to 2:0 V). The slightly larger window for the thin lm is not unexpected as the mobility of charge compensating counterions in the thin lm is far more restricted compared to the solution species. Small amounts of conduction are observed at the intervening potentials (−1:8 to +2:0 V), which we believe is associated with the capacitive charging of the lm as related to movement of the associated chloride anions. The capacitive charging current is observed to depend on the voltage scan rate. However, this e
ect is small because the electrode geometry of the capacitor is very small. Fig. 4 shows the observed di
erence in conductivity of identically prepared lms of the T-Ru6 and P-Ru6 hexamers, with the T-Ru6 isomer being more conductive. These hexamers are identical in all respects excepting for their ‘at bow tie’ (T) or ‘twisted bow tie’ (P) macroscopic shape or ‘tertiary structure’; suggesting that the di
ering tertiary structures give rise to di
ering 2-D arrays or packed assemblies which can ne tune the electrical properties. Fortunately, this tertiary structure is tunable by chemical synthesis adding a new and unusual level of control over the self-assembly process of such large molecules. Films of the more conductive T-Ru6 were also prepared on silicon wafers and examined for eld-modulated conductivity. Current leakage from the gate electrode to the source–drain electrodes was determined to be absent. Some unusual e
ects were observed as shown in Fig. 5. When the current vs. VSD was examined (shown in Fig. 5) two important e
ects were noted: (i) with each cycle there was a drift in the zero point indicating some type of lm charging and (ii) a plateau was observed in each run and its onset appears to be a function of the gate voltage applied (see Fig. 6). As the data obtained in Fig. 5 is for conduction at negative potentials, we postulate that the applied potential raises the LUMO level of the tpphz ligands with respect to ground, thus requiring more negative potentials to reductively dope these sites at large negative elds. A plot of the

131

90 80 70

0

1

2

3

VG

4

5

(V)

Fig. 7. E
ect of visible (white light) irradiation on the conductivity of T-Ru 6 lm. Open circles = light on. Filled circles = light o
. VSD = 8 V. Source was a tungsten lamp.

required voltage for onset of conduction (Vonset ) vs. the applied gate potential (VG ) is shown in Fig. 6 and supports this analysis. Finally, we observed an increased current when the lms were irradiated with white light. As shown in Fig. 7, the irradiated lm was more conductive than the non-irradiated lm, especially at low gate voltages (¡ 3 V). This e
ect is likely due to the photoinduced generation of additional charge carriers upon excitation into the Ru-polypyridyl MLCT transition, which has the e
ect of injecting electrons into the polypyridyl ligands LUMO and holes into the ruthenium-based HOMO. Clearly, these lms possess some unusual and interesting electrical and photonic behavior, however, a far more in depth and systematic study is needed to elucidate the underlying electronic and optical processes and mechanisms.

3. Conclusion This work helps reveal some unique properties of these molecules, which may render new applications in optical switching and modulation, probing of the electronic structure of single molecules, eld-e
ect devices, photovoltaic devices, and may demonstrate a potential pathway to the ultra large-scale integration of molecular devices. Importantly, these hybrid organic– inorganic molecules are considerably larger and more rigid than the purely organic molecules incorporated into molecular electronics thus far. The advantage of using these nanoscale building blocks (NBBs) is that

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fewer molecules are needed to complete the molecular assembly for bridging between electrical contacts, and therefore truly molecular properties may be accessed. Future e
orts will focus on modifying these NBBs with donor–acceptor endgroups to help direct the self-assembly process in a more controlled manner. Ultimately, such a strategy could lead to sophisticated molecular structures and circuits that could be self-assembled with lithography needed only for electrical contacts. Acknowledgements This work was supported in part by NSF and NASA Cooperative Agreement No. NCL-1-02038. References [1] A. Aviram, M.A. Ratner, Chem. Phys. Lett. 29 (1974) 277. [2] Y. Huang, X. Duan, Y. Cui, L.J. Lauhon, K.-H. Kim, C.M. Lieber, Science 294 (2001) 1313. [3] A. Bachtold, P. Hadley, T. Nakanishi, C. Dekker, Science 294 (2001) 1317. [4] D.L. Klein, R. Roth, A.K.L. Lim, A.P. Alivisatos, P.L. McEuen, Nature 389 (1997) 699. [5] D.L. Klein, P.L. McEuen, J.E.B. Katari, R. Roth, A.P. Alivisatos, Appl. Phys. Lett. 68 (1996) 2574. [6] Y. Cui, C.M. Lieber, Science 291 (2001) 851. [7] S.J. Tans, A.R.M. Verschueren, C. Dekker, Nature 393 (1998) 49. [8] M.A. Reed, C. Zhou, C.J. Muller, T.P. Burgin, J.M. Tour, Science 278 (1997) 252. [9] J. Chen, M.A. Reed, A.M. Rawlett, J.M. Tour, Science 286 (1999) 1550. [10] M.A. Reed, J. Chen, A.M. Rawlett, D.W. Price, J.M. Tour, Appl. Phys. Lett. 78 (2001) 3735. [11] W. Liang, M.P. Shores, M. Bockrath, J.R. Long, H. Park, Nature 417 (2002) 725.

[12] J. Park, A.N. Pasupathy, J.I. Goldsmith, C.K. Chang, Y. Yaish, J.R. Petta, M. Rinkoski, J.P. Sethna, H.D. Abruna, P.L. McEuen, D.C. Ralph, Nature 417 (2002) 722. [13] C. Zhou, M.R. Deshpande, M.A. Reed, L. Jones, J.M. Tour, Appl. Phys. Lett. 71 (1997) 611. [14] International Technology Roadmap for Semiconductors, Semiconductor Industry Association, New York, 2001. [15] K. Shinohara, Y. Yamashita, A. Endoh, K. Hikosaka, T. Matsui, T. Mimura, S. Hiyamizu, Jpn. J. Appl. Phys. 2 (41) (2002) L437. [16] S.J. Jeng, B. Jagannathan, J.S. Rieh, J. Johnson, K.T. Schonenberg, D. Greenberg, A. Stricker, H. Chen, M. Khater, D. Ahlgren, G. Freeman, K. Stein, S. Subbanna, IEEE Electron Device Lett. 22 (2001) 542. [17] C.P. Collier, E.W. Wong, M. Belohradsky, F.M. Raymo, J.F. Stoddart, P.J. Kuekes, R.S. Williams, J.R. Heath, Science 285 (1999) 391. [18] C.P. Collier, G. Mattersteig, E.W. Wong, Y. Luo, K. Beverly, J. Sampaio, F.M. Raymo, J.F. Stoddart, J.R. Heath, Science 289 (2000) 1172. [19] W.U. Huynh, J.J. Dittmer, A.P. Alivisatos, Science 295 (2002) 2425. [20] B. O’Regan, M. Gratzel, Nature 353 (1991) 737. [21] M. Schroder, T.A. Stepheson, in: G. Wilkinson, R.D. Gillard, J.A. McCleverty (Eds.), Comprehensive Coordination Chemistry, Vol. 4, Pergamon Books: Maxwell House, New York, 1987, p. 327. [22] V. Balzani, S. Campagna, G. Denti, A. Juris, S. Serroni, M. Venturi, Acc. Chem. Res. 31 (1998) 26. [23] F.G. Gao, A.J. Bard, J. Am. Chem. Soc. 122 (2000) 7426. [24] A.J. Bard, M.A. Fox, Acc. Chem. Res. 28 (1995) 141. [25] F.M. MacDonnell, M.M. Ali, M.-J. Kim, Comm. Inorg. Chem. 22 (2000) 203. [26] F.M. MacDonnell, M.-J. Kim, K.L. Wouters, R. Konduri, Hierarchical structure in assemblies of enantiopure ruthenium trisdiimine complexes: a biomimetic approach utilizing primary, secondary, teritary and quaternary structural elements, Coord. Chem. Rev. 2003, in press. [27] S. Campagna, S. Serroni, S. Bodige, F.M. MacDonnell, Inorg. Chem. 38 (1999) 692. [28] M.-J. Kim, F.M. MacDonnell, Unpublished results.