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Controlling the formation of gold nanoparticle domains onto inorganic substrates
ARTICLE IN PRESS
Physica E 40 (2008) 1709–1711 www.elsevier.com/locate/physe
Controlling the formation of gold nanoparticle domains onto inorganic substrates G. Canua,c,, M. Dipasqualeb,c, C. Berninic, P. Bonannob, E. Di Zittib, D. Marre`a,c, L. Pellegrinoa,c, D. Riccib,d, M. Sassettia,c, A.S. Siria,c b
a Department of Physics, University of Genova, via Dodecaneso 33, 16146 Genova, Italy Department of Biophysical and Electronic Engineering, University of Genova, via all’Opera Pia 11a, 16145 Genova, Italy c CNR-LAMIA, corso Perrone 24, 16152 Genova, Italy d IIT, via Morego 30, 16163 Genova, Italy
Available online 25 October 2007
Abstract The ability to control the spatial arrangement of molecular nanocrystals is of fundamental importance for the fabrication of nanoscale devices. In this work, we report on experimental procedures to control the formation of 50 nm, organically capped, gold nanoparticles by tuning the deposition conditions. We found the parameters which lead to the formation of different domain structures, from threedimensional aggregates to two-dimensional structures, to even isolated particles. r 2007 Elsevier B.V. All rights reserved. PACS: 81.07. b; 81.16. c; 81.16.Dn Keywords: Gold nanoparticles; Nanocrystals; Self-assembly; Nanostructured domains
1. Introduction It is widely recognized that self-assembly is an effective bottom-up strategy to construct nanodevices from molecular nanopatterns [1]. In this framework, thiol-stabilized nanoparticles (NPs) are attractive as functional materials for many applications, as their properties can be tuned by a careful choice of material, size, shape, environment and temperature [2]. In particular, they can be regarded as discrete entities that behave as quantum dots, as thiol ligands provide a suitable tunneling distance between the NPs, thus making them attractive nanomaterials to investigate novel quantum transport phenomena in confined systems [3]. However, alignment and stability of NP assembly, required to develop device structures, is still a demanding task. In this perspective, regular and controlled [4] Corresponding author. Department of Physics, University of Genova, via Dodecaneso 33, 16146 Genova, Italy. Tel.: +39 010 353 6323; fax: +39 010 311 066. E-mail address: canu@fisica.unige.it (G. Canu).
1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.10.061
assembly of NPs is of fundamental importance: this work addresses the experimental conditions to induce a selforganized growth of regular 2D gold NP structures, without any chemical modification of the substrate. Gold NPs of 50 nm diameter, functionalized with octanethiol, were employed. In this work we use different self-assembly deposition techniques to study how the shape and the size of spatial NP arrangements can be tuned by chemical conditions in order to make possible their exploitation in quantum nanostructures.
2. Experimental Gold NPs, bought from British Biocell Intl., were functionalized with octanethiol and dissolved in toluene, according to the procedure indicated by Weiss et al. [5]. Depositions were mainly performed onto silicon dioxide (SiO2). With the aim of integrating NP systems with functional materials, such as transition metal oxides, which have rich physical properties, and with the ultimate goal to bridge electrode gaps and to fabricate a device, we also
ARTICLE IN PRESS 1710
G. Canu et al. / Physica E 40 (2008) 1709–1711
deposited nanoparticles on single crystal substrates of strontium titanate (SrTiO3) as a model system. Both SiO2 and SrTiO3 were 5 mm 5 mm wide. SiO2 substrates were cleaned with acetone in an ultrasonic bath, dried in a nitrogen flux, and then baked in air at 120 1C for 30 min. SrTiO3 substrates were annealed in air at 900 1C for 12 h, in order to rearrange properly the surface and reduce impurities. Two different deposition techniques have been used to evaluate the influence of the NP solution volume: the drop casting method allowed to release 10 mL volume drops of NP solution, while a piezodropper system delivers much smaller drops (tenths of picoliters). The drop casting technique is a simple method to deliver drops of liquid in the micromolar to millimolar range. The drops are deposited mostly by means of micropipettes onto solid surfaces. This technique was applied in order to find the optimal concentration to induce two-dimensional (2D) NP structures, by varying the NP concentration, which was step-lowered from 4:5 1010 NPs/mL down to 4:5 108 NPs/mL. The piezoelectric droplet dispenser consists of a glass capillary embedded in a piezoceramic tube, fixed in a holding device [6]. Applying a voltage pulse to the piezoceramic tube, the tube and the capillary are contracted, enabling the droplet ejection. The piezodropper system was employed to better control the positioning of the NP domains, with the ultimate goal of placing NPs between an electrode pair. A micromanipulator system under an optical microscope was used for positioning the drop on the substrate. Depositions via the piezodropper dispenser were performed at a constant NP concentration of 4:5 1010 NPs/mL, while voltage magnitude and pulse duration were varied, in order to find the proper conditions for the formation of best quality NP assemblies. Their range was chosen from the calibration data provided by the manufacturer, indicating the delivering of a 79 mm size drop under the application of a voltage pulse amplitude of 64.7 V and 97 ms pulse duration. Deposition parameters were investigated using two different procedures: in the first one, the pulse duration was set at 200 ms and the voltage magnitude was varied in the range of 70–100 V, with 5 V steps; in the second one, the voltage magnitude was kept constant at 85 V and the pulse duration was varied in the 100–500 ms range. Several samples for each set of parameters were investigated; the results showed a high degree of reproducibility in all cases. Characterization of the NP assemblies was performed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). 3. Results and discussion The successful overcoating—octanethiol functionalization—of gold NPs was demonstrated by the ease of precipitate solubility in toluene (if not capped, the
Fig. 1. TEM image of octanethiol-functionalized gold NPs.
Fig. 2. SEM image of an NP monolayer fabricated by drop casting at the concentration of 4:5 109 NPs/mL.
precipitate would not redissolve). Fig. 1 shows a typical TEM image of the functionalized NPs, which have a diameter of 52:2 4:2 nm. Fig. 2 shows an SEM image of a NP assembly, obtained via drop casting. With a solution concentration of 4:5 109 NPs/mL, a monolayer zone more than 1 mm-wide formed. Similar results were obtained for all samples observed, on both SiO2 and SrTiO3 substrates. Evaporation of the drops took place in a few minutes. Concentration plays an important role in aggregation of NPs. By lowering the solution concentration, we observed a change in NP assembling from three-dimensional (3D) aggregates for 4:5 1010 NPs/mL up to even 1 mm-wide monolayers for 4:5 109 NPs/mL, to small islands formed by one or a few NPs for a concentration of 7 108 NPs/mL and, finally, 3D features again for 4:5 108 NPs/mL. As compared to the drop casting method, much smaller, and better positioned, drops can be delivered by using the piezodropper system. Ejected drops are tenths of picoliters, with consequent much smaller evaporation time, i.e. a few seconds.
ARTICLE IN PRESS G. Canu et al. / Physica E 40 (2008) 1709–1711
1711
This result can be explained in terms of NP behavior: NPs have enough mobility to move across the surface only in presence of liquid, i.e. toluene; the total amount of liquid dramatically changes (six orders of magnitude), while performing depositions with the two different techniques. Via drop casting, a lot of liquid is employed, resulting in a drying time of about 100 s, while the piezodropper system produces tiny drops, in such a way that the solvent evaporates in few seconds and NPs do not have enough time to aggregate in wider domains, even obtaining 2D structures. 4. Conclusions Fig. 3. SEM image of an NP 2D domain; NPs were deposited via the piezodropper system at the concentration of 4:5 108 NPs/mL.
The variation of voltage magnitude and pulse duration changes the arrangement of NPs on the substrate surface. Depositions performed at a constant pulse duration of 200 ms, with increasing the voltage magnitude from 70 V, brought to the primary formation of 3D structures, until the values of 85 and 90 V were reached; in these conditions NPs tend to aggregate in single-layer domains. Beyond these values, NPs tend to aggregate in three dimensions again. When the pulse duration was increased, keeping the voltage magnitude at 85 V, we observed a decrease in domain size: for 100 and 200 ms, we obtained about 500nm-wide domains, while for larger values we only found a few NP aggregates. Fig. 3 shows the SEM image of a typical monolayer obtained using the piezodropper system; most islands were arranged in regular 2D domains, when a 90 V voltage magnitude and a 200 ms pulse duration were applied. The piezodropper system, programmed with the correct parameters, provided much smaller 2D domains than those obtained by drop casting. In particular, most of the observed domains were islands no more than 500 nm wide.
In this work we studied the aggregation properties of 50 nm gold NPs, stabilized with octanethiol, through two different deposition techniques. By using the drop casting technique, an NP concentration which permitted to obtain more than 1 mm-wide 2D domains was found. A better control on the positioning of NPs on the substrates can be achieved using the piezodropper system, which enables the formation of hundreds of nanometers wide 2D domains. The possibility of tuning the length of NP domains opens the way to their exploitation as cost-effective quantum nanostructures between electrodes pairs, useful to perform quantum transport studies. References [1] B.A. Parviz, D. Ryan, G.M. Whitesides, IEEE Trans. Adv. Pack. 26 (2003) 233. [2] J. Love, L. Estroff, J. Kriebel, R. Nuzzo, G. Whitesides, Chem. Rev. 105 (2005) 1103. [3] M. Daniel, D. Astruc, Chem. Rev. 104 (2004) 293. [4] F. Iwata, S. Nagami, Y. Sumiya, A. Sasaki, Nanotechnology 18 (2007) 105301. [5] D.N. Weiss, X. Brokmann, L.E. Calvet, M.A. Kastner, M.G. Bawendi, Appl. Phys. Lett. 88 (2006) 143507. [6] E. Macis, M. Tedesco, P. Massobrio, R. Raiteri, S. Martinoia, J. Neurosci. Meth. 161 (2007) 88.
Physica E 40 (2008) 1709–1711 www.elsevier.com/locate/physe
Controlling the formation of gold nanoparticle domains onto inorganic substrates G. Canua,c,, M. Dipasqualeb,c, C. Berninic, P. Bonannob, E. Di Zittib, D. Marre`a,c, L. Pellegrinoa,c, D. Riccib,d, M. Sassettia,c, A.S. Siria,c b
a Department of Physics, University of Genova, via Dodecaneso 33, 16146 Genova, Italy Department of Biophysical and Electronic Engineering, University of Genova, via all’Opera Pia 11a, 16145 Genova, Italy c CNR-LAMIA, corso Perrone 24, 16152 Genova, Italy d IIT, via Morego 30, 16163 Genova, Italy
Available online 25 October 2007
Abstract The ability to control the spatial arrangement of molecular nanocrystals is of fundamental importance for the fabrication of nanoscale devices. In this work, we report on experimental procedures to control the formation of 50 nm, organically capped, gold nanoparticles by tuning the deposition conditions. We found the parameters which lead to the formation of different domain structures, from threedimensional aggregates to two-dimensional structures, to even isolated particles. r 2007 Elsevier B.V. All rights reserved. PACS: 81.07. b; 81.16. c; 81.16.Dn Keywords: Gold nanoparticles; Nanocrystals; Self-assembly; Nanostructured domains
1. Introduction It is widely recognized that self-assembly is an effective bottom-up strategy to construct nanodevices from molecular nanopatterns [1]. In this framework, thiol-stabilized nanoparticles (NPs) are attractive as functional materials for many applications, as their properties can be tuned by a careful choice of material, size, shape, environment and temperature [2]. In particular, they can be regarded as discrete entities that behave as quantum dots, as thiol ligands provide a suitable tunneling distance between the NPs, thus making them attractive nanomaterials to investigate novel quantum transport phenomena in confined systems [3]. However, alignment and stability of NP assembly, required to develop device structures, is still a demanding task. In this perspective, regular and controlled [4] Corresponding author. Department of Physics, University of Genova, via Dodecaneso 33, 16146 Genova, Italy. Tel.: +39 010 353 6323; fax: +39 010 311 066. E-mail address: canu@fisica.unige.it (G. Canu).
1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.10.061
assembly of NPs is of fundamental importance: this work addresses the experimental conditions to induce a selforganized growth of regular 2D gold NP structures, without any chemical modification of the substrate. Gold NPs of 50 nm diameter, functionalized with octanethiol, were employed. In this work we use different self-assembly deposition techniques to study how the shape and the size of spatial NP arrangements can be tuned by chemical conditions in order to make possible their exploitation in quantum nanostructures.
2. Experimental Gold NPs, bought from British Biocell Intl., were functionalized with octanethiol and dissolved in toluene, according to the procedure indicated by Weiss et al. [5]. Depositions were mainly performed onto silicon dioxide (SiO2). With the aim of integrating NP systems with functional materials, such as transition metal oxides, which have rich physical properties, and with the ultimate goal to bridge electrode gaps and to fabricate a device, we also
ARTICLE IN PRESS 1710
G. Canu et al. / Physica E 40 (2008) 1709–1711
deposited nanoparticles on single crystal substrates of strontium titanate (SrTiO3) as a model system. Both SiO2 and SrTiO3 were 5 mm 5 mm wide. SiO2 substrates were cleaned with acetone in an ultrasonic bath, dried in a nitrogen flux, and then baked in air at 120 1C for 30 min. SrTiO3 substrates were annealed in air at 900 1C for 12 h, in order to rearrange properly the surface and reduce impurities. Two different deposition techniques have been used to evaluate the influence of the NP solution volume: the drop casting method allowed to release 10 mL volume drops of NP solution, while a piezodropper system delivers much smaller drops (tenths of picoliters). The drop casting technique is a simple method to deliver drops of liquid in the micromolar to millimolar range. The drops are deposited mostly by means of micropipettes onto solid surfaces. This technique was applied in order to find the optimal concentration to induce two-dimensional (2D) NP structures, by varying the NP concentration, which was step-lowered from 4:5 1010 NPs/mL down to 4:5 108 NPs/mL. The piezoelectric droplet dispenser consists of a glass capillary embedded in a piezoceramic tube, fixed in a holding device [6]. Applying a voltage pulse to the piezoceramic tube, the tube and the capillary are contracted, enabling the droplet ejection. The piezodropper system was employed to better control the positioning of the NP domains, with the ultimate goal of placing NPs between an electrode pair. A micromanipulator system under an optical microscope was used for positioning the drop on the substrate. Depositions via the piezodropper dispenser were performed at a constant NP concentration of 4:5 1010 NPs/mL, while voltage magnitude and pulse duration were varied, in order to find the proper conditions for the formation of best quality NP assemblies. Their range was chosen from the calibration data provided by the manufacturer, indicating the delivering of a 79 mm size drop under the application of a voltage pulse amplitude of 64.7 V and 97 ms pulse duration. Deposition parameters were investigated using two different procedures: in the first one, the pulse duration was set at 200 ms and the voltage magnitude was varied in the range of 70–100 V, with 5 V steps; in the second one, the voltage magnitude was kept constant at 85 V and the pulse duration was varied in the 100–500 ms range. Several samples for each set of parameters were investigated; the results showed a high degree of reproducibility in all cases. Characterization of the NP assemblies was performed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). 3. Results and discussion The successful overcoating—octanethiol functionalization—of gold NPs was demonstrated by the ease of precipitate solubility in toluene (if not capped, the
Fig. 1. TEM image of octanethiol-functionalized gold NPs.
Fig. 2. SEM image of an NP monolayer fabricated by drop casting at the concentration of 4:5 109 NPs/mL.
precipitate would not redissolve). Fig. 1 shows a typical TEM image of the functionalized NPs, which have a diameter of 52:2 4:2 nm. Fig. 2 shows an SEM image of a NP assembly, obtained via drop casting. With a solution concentration of 4:5 109 NPs/mL, a monolayer zone more than 1 mm-wide formed. Similar results were obtained for all samples observed, on both SiO2 and SrTiO3 substrates. Evaporation of the drops took place in a few minutes. Concentration plays an important role in aggregation of NPs. By lowering the solution concentration, we observed a change in NP assembling from three-dimensional (3D) aggregates for 4:5 1010 NPs/mL up to even 1 mm-wide monolayers for 4:5 109 NPs/mL, to small islands formed by one or a few NPs for a concentration of 7 108 NPs/mL and, finally, 3D features again for 4:5 108 NPs/mL. As compared to the drop casting method, much smaller, and better positioned, drops can be delivered by using the piezodropper system. Ejected drops are tenths of picoliters, with consequent much smaller evaporation time, i.e. a few seconds.
ARTICLE IN PRESS G. Canu et al. / Physica E 40 (2008) 1709–1711
1711
This result can be explained in terms of NP behavior: NPs have enough mobility to move across the surface only in presence of liquid, i.e. toluene; the total amount of liquid dramatically changes (six orders of magnitude), while performing depositions with the two different techniques. Via drop casting, a lot of liquid is employed, resulting in a drying time of about 100 s, while the piezodropper system produces tiny drops, in such a way that the solvent evaporates in few seconds and NPs do not have enough time to aggregate in wider domains, even obtaining 2D structures. 4. Conclusions Fig. 3. SEM image of an NP 2D domain; NPs were deposited via the piezodropper system at the concentration of 4:5 108 NPs/mL.
The variation of voltage magnitude and pulse duration changes the arrangement of NPs on the substrate surface. Depositions performed at a constant pulse duration of 200 ms, with increasing the voltage magnitude from 70 V, brought to the primary formation of 3D structures, until the values of 85 and 90 V were reached; in these conditions NPs tend to aggregate in single-layer domains. Beyond these values, NPs tend to aggregate in three dimensions again. When the pulse duration was increased, keeping the voltage magnitude at 85 V, we observed a decrease in domain size: for 100 and 200 ms, we obtained about 500nm-wide domains, while for larger values we only found a few NP aggregates. Fig. 3 shows the SEM image of a typical monolayer obtained using the piezodropper system; most islands were arranged in regular 2D domains, when a 90 V voltage magnitude and a 200 ms pulse duration were applied. The piezodropper system, programmed with the correct parameters, provided much smaller 2D domains than those obtained by drop casting. In particular, most of the observed domains were islands no more than 500 nm wide.
In this work we studied the aggregation properties of 50 nm gold NPs, stabilized with octanethiol, through two different deposition techniques. By using the drop casting technique, an NP concentration which permitted to obtain more than 1 mm-wide 2D domains was found. A better control on the positioning of NPs on the substrates can be achieved using the piezodropper system, which enables the formation of hundreds of nanometers wide 2D domains. The possibility of tuning the length of NP domains opens the way to their exploitation as cost-effective quantum nanostructures between electrodes pairs, useful to perform quantum transport studies. References [1] B.A. Parviz, D. Ryan, G.M. Whitesides, IEEE Trans. Adv. Pack. 26 (2003) 233. [2] J. Love, L. Estroff, J. Kriebel, R. Nuzzo, G. Whitesides, Chem. Rev. 105 (2005) 1103. [3] M. Daniel, D. Astruc, Chem. Rev. 104 (2004) 293. [4] F. Iwata, S. Nagami, Y. Sumiya, A. Sasaki, Nanotechnology 18 (2007) 105301. [5] D.N. Weiss, X. Brokmann, L.E. Calvet, M.A. Kastner, M.G. Bawendi, Appl. Phys. Lett. 88 (2006) 143507. [6] E. Macis, M. Tedesco, P. Massobrio, R. Raiteri, S. Martinoia, J. Neurosci. Meth. 161 (2007) 88.