Letter pubs.acs.org/NanoLett
A Thermal Plasmonic Sensor Platform: Resistive Heating of Nanohole Arrays Mudassar Virk,† Kunli Xiong, Mikael Svedendahl, Mikael Kal̈ l, and Andreas B. Dahlin* Department of Applied Physics, Chalmers University of Technology, Göteborg, Sweden S Supporting Information *
ABSTRACT: We have created a simple and efficient thermal plasmonic sensor platform by letting a DC current heat plasmonic nanohole arrays. The sensor can be used to determine thermodynamic parameters in addition to monitoring molecular reactions in real-time. As an application example, we use the thermal sensor to determine the kinetics and activation energy for desorption of thiol monolayers on gold. Further, the temperature of the metal can be measured optically by the spectral shift of the bonding surface plasmon mode (0.015 nm/ K). We show that this resonance shift is caused by thermal lattice expansion, which reduces the plasma frequency of the metal. The sensor is also used to determine the thin film thermal expansion coefficient through a theoretical model for the expected resonance shift. KEYWORDS: Plasmon, nanohole, gold, temperature, current, thermal expansion
P
current can be used to heat plasmonic sensor surfaces. We show how nanohole arrays in thin gold films are excellent structures to heat in this manner. The resistive heating makes it possible to study fundamental aspects of materials science, such as thermal expansion in thin films. We also show how our thermal plasmonic sensor platform can be used to investigate the temperature dependence of molecular reactions on surfaces and extract thermodynamic parameters. The thermal plasmonic sensor is illustrated in Figure 1. A borosilicate support (glass transition ∼800 K) is fully covered with a plasmonic nanohole array consisting of ∼150 nm shortrange ordered holes in a 30 nm Au film, fabricated as described previously.18,19 A highly conductive paste (Ag epoxy) is placed along the short edges of the sample, which are then connected to a power supply. In this manner, the voltage is kept almost constant along the short edges (variation ∼5% of voltage across the sample), thereby providing a more homogeneous heating of the nanohole arrays. For some calibration experiments (see below), a thermal camera is used to acquire the absolute sample temperature and its distribution. The extinction spectrum is measured from an ∼5 mm spot approximately in the middle of the sample. The nanohole arrays used here are ideal for resistive heating primarily due to their resistance, which was typically R = 3 Ω across the sample. The heat power is given by P = UI = U2/R and thus for a given voltage source one wants low resistance to reach high temperatures. However, the resistance should not be too low because it will eventually become comparable to that of cables and contact points in the circuit in which case the heating no longer occurs on the sensor surface.
lasmonics deals with understanding and controlling electromagnetic fields on the nanoscale through the use of metallic nanostructures. Recently, a subtopic referred to as thermoplasmonics has emerged.1 Thermoplasmonics is based on utilizing light from artificial2 or natural3 sources absorbed by plasmon resonances to control temperatures on the nanoscale.4 Some examples of applications in thermoplasmonics are convection control,5 cancer treatment,6 water sterilization,3 and lithography.7 In addition, heating a plasmonic nanostructure is of great interest in a technical sense for the incorporation of temperature control in nanoplasmonic sensors for materials science8 or biology.9 This is a prerequisite to determine thermodynamic parameters. However, achieving significant heating of plasmonic sensor surfaces is not straightforward when utilizing the energy of the incident light. Although single nanoparticles can be heated by high power coherent sources focused to diffraction limited spots,2,10,11 such approaches fail for the majority of plasmonic sensors because they rely on white probing light and larger sensor areas.12 Crude approaches such as putting the whole measurement system in an oven of some sort are feasible but provide very slow heating/cooling13 as well as complications when more than just the sensor surface is heated. Also, it is far from straightforward to accurately control and measure the temperature just at the metal surface,2,14 where reactions of interest are detected with plasmonic sensors. Furthermore, despite the interest in thermoplasmonics there are few studies on the fundamental influence from metal temperature on the optical response. Existing work has focused on the reduction in resonance width by cooling15,16 and suffers from difficulty of separating the intrinsic temperature effect, that is, the heating of the metal, from other effects such as changes in refractive index (RI) of the surrounding material.17 In this work, we show how a DC © XXXX American Chemical Society
Received: March 28, 2014 Revised: April 28, 2014
A
dx.doi.org/10.1021/nl5011542 | Nano Lett. XXXX, XXX, XXX−XXX
Nano Letters
Letter
of a conductor. We also tested resistive heating of Au nanodisks on indium tin oxide18,19 with successful results. However, the relatively high resistance of those surfaces imposed much higher voltages (around a factor of 100) in order to reach the same temperatures, making such a thermal plasmonic sensor platform less practical. In order to calibrate the temperature increase on the sensor surface and to image the temperature distribution we used a thermal camera21 (Figure 2A). The dynamics of the resistive heating process and the temperature distribution were also simulated in COMSOL Multiphysics for the whole sample (glass and thin film). A comparison between camera readout and simulation results is shown in Figure 2A for the case of I = 0.7 A (U ≈ 2 V). The calculated heating dynamics follow the camera readout almost perfectly and a temperature of approximately 380 K is reached in ∼1 min starting from room temperature (298 K). Notably, the heat distribution at steady state is quite homogeneous across the sample (Figure 2A). The simulated temperature is very similar although more homogeneous than that observed experimentally, which can be related to the conductive paste and the metal clamps holding the sample (not accounted for in simulations). Note that due to the well-known and high emissivity of glass the camera imaged the backside of the sample (Figure 1) in order to get accurate results, because the emissivity of metals is too poor. We verified that the glass backside had the same temperature as the nanostructure side by using a thermocouple22 (data in Supporting Information). However, the thermocouple alone could not provide an accurate absolute surface temperature because physically contacting the surface by itself influences the local temperature. Still, a relative comparison between both sides of the samples was possible. Also, the simulations verified that the glass backside had the same surface temperature as the gold nanostructure (difference 0.999, indistinguishable from adjusted R2), that is, Δλpeak = A1exp(−k1t) + A2 exp(−k2t) (red dashed curve). We extracted the constants to A1 = 1.1 nm and k1 = 0.063 min−1 as well as A2 = 1.5 nm and k2 = 0.0071 min−1 for the data in Figure 4. It was not possible to describe data of desorption with a single exponential decay, supporting that there are two reaction mechanisms by which thiol desorption can occur. Further, it has been an unresolved issue in the literature if there is a lag time before the slower desorption mechanism initiates.33 Our kinetics analysis shows that this seems unlikely since the decay curve could not be well fitted to such a model. Finally, we note that the total blue shift from SAM desorption converges to approximately 2.7 nm, which is in fact a bit lower than the red shift measured upon SAM formation. We measured the signal from the SAM by comparing spectra in air before and after binding of MUA, which gave an extinction peak shift of 3.3 ± 0.1 nm. This suggest that ∼20% of the thiols are not possible to remove by heating. When heating SAMs to higher temperatures we observed faster desorption kinetics as expected but the same fraction of the SAM seemed to remain on the surface still. Similar behavior has been reported for other SAMs.34 The possibility to easily control temperature and measure molecular binding/unbinding in real-time also makes it possible to enhance reaction rates and determine thermodynamic parameters in addition to the rate constants. The Eyring equation relates rate constant and activation energy E* as k = kBT/h exp(−E*/(kBT)), where kB is Boltzmann’s constant and
■
ASSOCIATED CONTENT
S Supporting Information *
A description of the model for gold permittivity at elevated temperatures, thermocouple data, a list of insignificant contributions to the plasmonic signal, experimental details, characteristic spacing of the arrays, and thermal stability limits. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Present Address †
(M.V.) Department of Nanobiotechnology, University of Natural Resources and Life Sciences Vienna.
E
dx.doi.org/10.1021/nl5011542 | Nano Lett. XXXX, XXX, XXX−XXX
Nano Letters
Letter
Author Contributions
(24) Ikenoya, Y.; Susa, M.; Shi, J.; Nakamura, Y.; Dahlin, A. B.; Sannomiya, T. J. Phys. Chem. C 2013, 117, 6373−6382. (25) Dahlin, A. B.; Mapar, M.; Xiong, K.; Mazzotta, F.; Hook, F.; Sannomiya, T. Adv. Opt. Mater. 2014, DOI: 10.1002/ adom.201300510. (26) Junesch, J.; Sannomiya, T.; Dahlin, A. B. ACS Nano 2012, 6, 10405−10415. (27) Sannomiya, T.; Scholder, O.; Jefimovs, K.; Hafner, C.; Dahlin, A. B. Small 2011, 7, 1653−1663. (28) Schwind, M.; Kasemo, B.; Zoric, I. Nano Lett. 2013, 13, 1743− 1750. (29) Etchegoin, P. G.; Le Ru, E. C.; Meyer, M. J. Chem. Phys. 2006, 125, 164705. (30) Fang, W. L.; Lo, C. Y. Sens. Actuators, A 2000, 84, 310−314. (31) Vericat, C.; Vela, M. E.; Benitez, G.; Carro, P.; Salvarezza, R. C. Chem. Soc. Rev. 2010, 39, 1805−1834. (32) Garg, N.; Carrasquillo-Molina, E.; Lee, T. R. Langmuir 2002, 18, 2717−2726. (33) Shadnam, M. R.; Amirfazli, A. Chem. Commun. 2005, 4869− 4871. (34) Shon, Y. S.; Lee, T. R. J. Phys. Chem. B 2000, 104, 8192−8200. (35) Tucceri, R. Surf. Sci. Rep. 2004, 56, 85−157. (36) Barik, A.; Otto, L. M.; Yoo, D.; Jose, J.; Johnson, T. W.; Oh, S.H. Nano Lett. 2014, 14, 2006−2012. (37) Dahlin, A. B.; Dielacher, B.; Rajendran, P.; Sugihara, K.; Sannomiya, T.; Zenobi-Wong, M.; Voros, J. Anal. Bioanal. Chem. 2012, 402, 1773−1784.
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. M.V. and K.X. contributed equally. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS Six undergraduate students are kindly acknowledged for contributing to this work through their thesis work: Pontus Andersson, Filippa Borg, Jens Carlsson, Robert Forslund, YeYuan Josefsson, and Tian-Fu Yun. This work was funded by the Swedish Foundation for Strategic Research, the Swedish Research Council, Knut & Alice Wallenberg Foundation, and the EU FP7 Program (Marie Curie Career Integration Grant).
■ ■
ABBREVIATIONS SPP, surface plasmon polariton; RI, refractive index; SAM, selfassembled monolayer; MUA, mercaptoundecanoic acid REFERENCES
(1) Baffou, G.; Quidant, R. Laser Photonics Rev. 2012, 7, 1−17. (2) Fang, Z. Y.; Zhen, Y. R.; Neumann, O.; Polman, A.; de Abajo, F. J. G.; Nordlander, P.; Halas, N. J. Nano Lett. 2013, 13, 1736−1742. (3) Neumann, O.; Urban, A. S.; Day, J.; Lal, S.; Nordlander, P.; Halas, N. J. ACS Nano 2013, 7, 42−49. (4) Baffou, G.; Girard, C.; Quidant, R. Phys. Rev. Lett. 2010, 104, 136805. (5) Donner, J. S.; Baffou, G.; McCloskey, D.; Quidant, R. ACS Nano 2011, 5, 5457−5462. (6) Hirsch, L. R.; Stafford, R. J.; Bankson, J. A.; Sershen, S. R.; Rivera, B.; Price, R. E.; Hazle, J. D.; Halas, N. J.; West, J. L. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 13549−13554. (7) Fedoruk, M.; Meixner, M.; Carretero-Palacios, S.; Lohmuller, T.; Feldmann, J. ACS Nano 2013, 7, 7648−7653. (8) Larsson, E. M.; Syrenova, S.; Langhammer, C. Nanophotonics 2012, 1, 249−266. (9) Dahlin, A. B.; Wittenberg, N. J.; Hook, F.; Oh, S.-H. Nanophotonics 2013, 2, 83−101. (10) Herzog, J. B.; Knight, M. W.; Natelson, D. Nano Lett. 2014, 14, 499−503. (11) Zijlstra, P.; Paulo, P. M. R.; Orrit, M. Nat. Nanotechnol. 2012, 7, 379−382. (12) Baffou, G.; Berto, P.; Urena, E. B.; Quidant, R.; Monneret, S.; Polleux, J.; Rigneault, H. ACS Nano 2013, 7, 6478−6488. (13) Langhammer, C.; Larsson, E. M.; Kasemo, B.; Zoric, I. Nano Lett. 2010, 10, 3529−3538. (14) Jonsson, M. P.; Dekker, C. Nano Lett. 2013, 13, 1029−1033. (15) Bouillard, J. S. G.; Dickson, W.; O’Connor, D. P.; Wurtz, G. A.; Zayats, A. V. Nano Lett. 2012, 12, 1561−1565. (16) Liu, M. Z.; Pelton, M.; Guyot-Sionnest, P. Phys. Rev. B 2009, 79, 035418. (17) Yeshchenko, O. A.; Bondarchuk, I. S.; Gurin, V. S.; Dmitruk, I. M.; Kotko, A. V. Surf. Sci. 2013, 608, 275−281. (18) Dahlin, A. B.; Sannomiya, T.; Zahn, R.; Sotiriou, G. A.; Voros, J. Nano Lett. 2011, 11, 1337−1343. (19) Dahlin, A. B.; Zahn, R.; Voros, J. Nanoscale 2012, 4, 2339−2351. (20) Reilly, T. H.; Tenent, R. C.; Barnes, T. M.; Rowlen, K. L.; van de Lagemaat, J. ACS Nano 2010, 4, 615−624. (21) Svedendahl, M.; Johansson, P.; Kall, M. Nano Lett. 2013, 13, 3053−3058. (22) Aviles, F.; Ceh, O.; Oliva, A. I. Surf. Rev. Lett. 2005, 12, 101− 106. (23) Ma, W. G.; Zhang, X.; Takahashi, K. J. Phys. D: Appl. Phys. 2010, 43, 465301. F
dx.doi.org/10.1021/nl5011542 | Nano Lett. XXXX, XXX, XXX−XXX
A Thermal Plasmonic Sensor Platform: Resistive Heating of Nanohole Arrays Mudassar Virk,† Kunli Xiong, Mikael Svedendahl, Mikael Kal̈ l, and Andreas B. Dahlin* Department of Applied Physics, Chalmers University of Technology, Göteborg, Sweden S Supporting Information *
ABSTRACT: We have created a simple and efficient thermal plasmonic sensor platform by letting a DC current heat plasmonic nanohole arrays. The sensor can be used to determine thermodynamic parameters in addition to monitoring molecular reactions in real-time. As an application example, we use the thermal sensor to determine the kinetics and activation energy for desorption of thiol monolayers on gold. Further, the temperature of the metal can be measured optically by the spectral shift of the bonding surface plasmon mode (0.015 nm/ K). We show that this resonance shift is caused by thermal lattice expansion, which reduces the plasma frequency of the metal. The sensor is also used to determine the thin film thermal expansion coefficient through a theoretical model for the expected resonance shift. KEYWORDS: Plasmon, nanohole, gold, temperature, current, thermal expansion
P
current can be used to heat plasmonic sensor surfaces. We show how nanohole arrays in thin gold films are excellent structures to heat in this manner. The resistive heating makes it possible to study fundamental aspects of materials science, such as thermal expansion in thin films. We also show how our thermal plasmonic sensor platform can be used to investigate the temperature dependence of molecular reactions on surfaces and extract thermodynamic parameters. The thermal plasmonic sensor is illustrated in Figure 1. A borosilicate support (glass transition ∼800 K) is fully covered with a plasmonic nanohole array consisting of ∼150 nm shortrange ordered holes in a 30 nm Au film, fabricated as described previously.18,19 A highly conductive paste (Ag epoxy) is placed along the short edges of the sample, which are then connected to a power supply. In this manner, the voltage is kept almost constant along the short edges (variation ∼5% of voltage across the sample), thereby providing a more homogeneous heating of the nanohole arrays. For some calibration experiments (see below), a thermal camera is used to acquire the absolute sample temperature and its distribution. The extinction spectrum is measured from an ∼5 mm spot approximately in the middle of the sample. The nanohole arrays used here are ideal for resistive heating primarily due to their resistance, which was typically R = 3 Ω across the sample. The heat power is given by P = UI = U2/R and thus for a given voltage source one wants low resistance to reach high temperatures. However, the resistance should not be too low because it will eventually become comparable to that of cables and contact points in the circuit in which case the heating no longer occurs on the sensor surface.
lasmonics deals with understanding and controlling electromagnetic fields on the nanoscale through the use of metallic nanostructures. Recently, a subtopic referred to as thermoplasmonics has emerged.1 Thermoplasmonics is based on utilizing light from artificial2 or natural3 sources absorbed by plasmon resonances to control temperatures on the nanoscale.4 Some examples of applications in thermoplasmonics are convection control,5 cancer treatment,6 water sterilization,3 and lithography.7 In addition, heating a plasmonic nanostructure is of great interest in a technical sense for the incorporation of temperature control in nanoplasmonic sensors for materials science8 or biology.9 This is a prerequisite to determine thermodynamic parameters. However, achieving significant heating of plasmonic sensor surfaces is not straightforward when utilizing the energy of the incident light. Although single nanoparticles can be heated by high power coherent sources focused to diffraction limited spots,2,10,11 such approaches fail for the majority of plasmonic sensors because they rely on white probing light and larger sensor areas.12 Crude approaches such as putting the whole measurement system in an oven of some sort are feasible but provide very slow heating/cooling13 as well as complications when more than just the sensor surface is heated. Also, it is far from straightforward to accurately control and measure the temperature just at the metal surface,2,14 where reactions of interest are detected with plasmonic sensors. Furthermore, despite the interest in thermoplasmonics there are few studies on the fundamental influence from metal temperature on the optical response. Existing work has focused on the reduction in resonance width by cooling15,16 and suffers from difficulty of separating the intrinsic temperature effect, that is, the heating of the metal, from other effects such as changes in refractive index (RI) of the surrounding material.17 In this work, we show how a DC © XXXX American Chemical Society
Received: March 28, 2014 Revised: April 28, 2014
A
dx.doi.org/10.1021/nl5011542 | Nano Lett. XXXX, XXX, XXX−XXX
Nano Letters
Letter
of a conductor. We also tested resistive heating of Au nanodisks on indium tin oxide18,19 with successful results. However, the relatively high resistance of those surfaces imposed much higher voltages (around a factor of 100) in order to reach the same temperatures, making such a thermal plasmonic sensor platform less practical. In order to calibrate the temperature increase on the sensor surface and to image the temperature distribution we used a thermal camera21 (Figure 2A). The dynamics of the resistive heating process and the temperature distribution were also simulated in COMSOL Multiphysics for the whole sample (glass and thin film). A comparison between camera readout and simulation results is shown in Figure 2A for the case of I = 0.7 A (U ≈ 2 V). The calculated heating dynamics follow the camera readout almost perfectly and a temperature of approximately 380 K is reached in ∼1 min starting from room temperature (298 K). Notably, the heat distribution at steady state is quite homogeneous across the sample (Figure 2A). The simulated temperature is very similar although more homogeneous than that observed experimentally, which can be related to the conductive paste and the metal clamps holding the sample (not accounted for in simulations). Note that due to the well-known and high emissivity of glass the camera imaged the backside of the sample (Figure 1) in order to get accurate results, because the emissivity of metals is too poor. We verified that the glass backside had the same temperature as the nanostructure side by using a thermocouple22 (data in Supporting Information). However, the thermocouple alone could not provide an accurate absolute surface temperature because physically contacting the surface by itself influences the local temperature. Still, a relative comparison between both sides of the samples was possible. Also, the simulations verified that the glass backside had the same surface temperature as the gold nanostructure (difference 0.999, indistinguishable from adjusted R2), that is, Δλpeak = A1exp(−k1t) + A2 exp(−k2t) (red dashed curve). We extracted the constants to A1 = 1.1 nm and k1 = 0.063 min−1 as well as A2 = 1.5 nm and k2 = 0.0071 min−1 for the data in Figure 4. It was not possible to describe data of desorption with a single exponential decay, supporting that there are two reaction mechanisms by which thiol desorption can occur. Further, it has been an unresolved issue in the literature if there is a lag time before the slower desorption mechanism initiates.33 Our kinetics analysis shows that this seems unlikely since the decay curve could not be well fitted to such a model. Finally, we note that the total blue shift from SAM desorption converges to approximately 2.7 nm, which is in fact a bit lower than the red shift measured upon SAM formation. We measured the signal from the SAM by comparing spectra in air before and after binding of MUA, which gave an extinction peak shift of 3.3 ± 0.1 nm. This suggest that ∼20% of the thiols are not possible to remove by heating. When heating SAMs to higher temperatures we observed faster desorption kinetics as expected but the same fraction of the SAM seemed to remain on the surface still. Similar behavior has been reported for other SAMs.34 The possibility to easily control temperature and measure molecular binding/unbinding in real-time also makes it possible to enhance reaction rates and determine thermodynamic parameters in addition to the rate constants. The Eyring equation relates rate constant and activation energy E* as k = kBT/h exp(−E*/(kBT)), where kB is Boltzmann’s constant and
■
ASSOCIATED CONTENT
S Supporting Information *
A description of the model for gold permittivity at elevated temperatures, thermocouple data, a list of insignificant contributions to the plasmonic signal, experimental details, characteristic spacing of the arrays, and thermal stability limits. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Present Address †
(M.V.) Department of Nanobiotechnology, University of Natural Resources and Life Sciences Vienna.
E
dx.doi.org/10.1021/nl5011542 | Nano Lett. XXXX, XXX, XXX−XXX
Nano Letters
Letter
Author Contributions
(24) Ikenoya, Y.; Susa, M.; Shi, J.; Nakamura, Y.; Dahlin, A. B.; Sannomiya, T. J. Phys. Chem. C 2013, 117, 6373−6382. (25) Dahlin, A. B.; Mapar, M.; Xiong, K.; Mazzotta, F.; Hook, F.; Sannomiya, T. Adv. Opt. Mater. 2014, DOI: 10.1002/ adom.201300510. (26) Junesch, J.; Sannomiya, T.; Dahlin, A. B. ACS Nano 2012, 6, 10405−10415. (27) Sannomiya, T.; Scholder, O.; Jefimovs, K.; Hafner, C.; Dahlin, A. B. Small 2011, 7, 1653−1663. (28) Schwind, M.; Kasemo, B.; Zoric, I. Nano Lett. 2013, 13, 1743− 1750. (29) Etchegoin, P. G.; Le Ru, E. C.; Meyer, M. J. Chem. Phys. 2006, 125, 164705. (30) Fang, W. L.; Lo, C. Y. Sens. Actuators, A 2000, 84, 310−314. (31) Vericat, C.; Vela, M. E.; Benitez, G.; Carro, P.; Salvarezza, R. C. Chem. Soc. Rev. 2010, 39, 1805−1834. (32) Garg, N.; Carrasquillo-Molina, E.; Lee, T. R. Langmuir 2002, 18, 2717−2726. (33) Shadnam, M. R.; Amirfazli, A. Chem. Commun. 2005, 4869− 4871. (34) Shon, Y. S.; Lee, T. R. J. Phys. Chem. B 2000, 104, 8192−8200. (35) Tucceri, R. Surf. Sci. Rep. 2004, 56, 85−157. (36) Barik, A.; Otto, L. M.; Yoo, D.; Jose, J.; Johnson, T. W.; Oh, S.H. Nano Lett. 2014, 14, 2006−2012. (37) Dahlin, A. B.; Dielacher, B.; Rajendran, P.; Sugihara, K.; Sannomiya, T.; Zenobi-Wong, M.; Voros, J. Anal. Bioanal. Chem. 2012, 402, 1773−1784.
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. M.V. and K.X. contributed equally. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS Six undergraduate students are kindly acknowledged for contributing to this work through their thesis work: Pontus Andersson, Filippa Borg, Jens Carlsson, Robert Forslund, YeYuan Josefsson, and Tian-Fu Yun. This work was funded by the Swedish Foundation for Strategic Research, the Swedish Research Council, Knut & Alice Wallenberg Foundation, and the EU FP7 Program (Marie Curie Career Integration Grant).
■ ■
ABBREVIATIONS SPP, surface plasmon polariton; RI, refractive index; SAM, selfassembled monolayer; MUA, mercaptoundecanoic acid REFERENCES
(1) Baffou, G.; Quidant, R. Laser Photonics Rev. 2012, 7, 1−17. (2) Fang, Z. Y.; Zhen, Y. R.; Neumann, O.; Polman, A.; de Abajo, F. J. G.; Nordlander, P.; Halas, N. J. Nano Lett. 2013, 13, 1736−1742. (3) Neumann, O.; Urban, A. S.; Day, J.; Lal, S.; Nordlander, P.; Halas, N. J. ACS Nano 2013, 7, 42−49. (4) Baffou, G.; Girard, C.; Quidant, R. Phys. Rev. Lett. 2010, 104, 136805. (5) Donner, J. S.; Baffou, G.; McCloskey, D.; Quidant, R. ACS Nano 2011, 5, 5457−5462. (6) Hirsch, L. R.; Stafford, R. J.; Bankson, J. A.; Sershen, S. R.; Rivera, B.; Price, R. E.; Hazle, J. D.; Halas, N. J.; West, J. L. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 13549−13554. (7) Fedoruk, M.; Meixner, M.; Carretero-Palacios, S.; Lohmuller, T.; Feldmann, J. ACS Nano 2013, 7, 7648−7653. (8) Larsson, E. M.; Syrenova, S.; Langhammer, C. Nanophotonics 2012, 1, 249−266. (9) Dahlin, A. B.; Wittenberg, N. J.; Hook, F.; Oh, S.-H. Nanophotonics 2013, 2, 83−101. (10) Herzog, J. B.; Knight, M. W.; Natelson, D. Nano Lett. 2014, 14, 499−503. (11) Zijlstra, P.; Paulo, P. M. R.; Orrit, M. Nat. Nanotechnol. 2012, 7, 379−382. (12) Baffou, G.; Berto, P.; Urena, E. B.; Quidant, R.; Monneret, S.; Polleux, J.; Rigneault, H. ACS Nano 2013, 7, 6478−6488. (13) Langhammer, C.; Larsson, E. M.; Kasemo, B.; Zoric, I. Nano Lett. 2010, 10, 3529−3538. (14) Jonsson, M. P.; Dekker, C. Nano Lett. 2013, 13, 1029−1033. (15) Bouillard, J. S. G.; Dickson, W.; O’Connor, D. P.; Wurtz, G. A.; Zayats, A. V. Nano Lett. 2012, 12, 1561−1565. (16) Liu, M. Z.; Pelton, M.; Guyot-Sionnest, P. Phys. Rev. B 2009, 79, 035418. (17) Yeshchenko, O. A.; Bondarchuk, I. S.; Gurin, V. S.; Dmitruk, I. M.; Kotko, A. V. Surf. Sci. 2013, 608, 275−281. (18) Dahlin, A. B.; Sannomiya, T.; Zahn, R.; Sotiriou, G. A.; Voros, J. Nano Lett. 2011, 11, 1337−1343. (19) Dahlin, A. B.; Zahn, R.; Voros, J. Nanoscale 2012, 4, 2339−2351. (20) Reilly, T. H.; Tenent, R. C.; Barnes, T. M.; Rowlen, K. L.; van de Lagemaat, J. ACS Nano 2010, 4, 615−624. (21) Svedendahl, M.; Johansson, P.; Kall, M. Nano Lett. 2013, 13, 3053−3058. (22) Aviles, F.; Ceh, O.; Oliva, A. I. Surf. Rev. Lett. 2005, 12, 101− 106. (23) Ma, W. G.; Zhang, X.; Takahashi, K. J. Phys. D: Appl. Phys. 2010, 43, 465301. F
dx.doi.org/10.1021/nl5011542 | Nano Lett. XXXX, XXX, XXX−XXX
