Self Assembled Materials - Revision history

Cmditradmin at 21:56, 23 September 2009

2009-09-23T21:56:05Z

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	'''Influence of organic thiols on the work function of coinage metals'''		'''Influence of organic thiols on the work function of coinage metals'''

			Contributed by Michal Malicki - Georgia Tech- Thesis Introduction

	A critical factor in self assembled materials concerns the [[Work Function of Metals]].		A critical factor in self assembled materials concerns the [[Work Function of Metals]].

Cmditradmin at 21:50, 23 September 2009

2009-09-23T21:50:46Z

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Cmditradmin: /* Work Function of Metals – Influence of Adsorbates */

2009-09-23T19:55:03Z

Work Function of Metals – Influence of Adsorbates

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	This results in the formation of a sheet of dipoles in the space occupied by the adsorbed atoms which alters the potential landscape at the vacuum / metal interface.<sup>12,13'''</sup>		This results in the formation of a sheet of dipoles in the space occupied by the adsorbed atoms which alters the potential landscape at the vacuum / metal interface.<sup>12,13'''</sup>

	On the basis of simple electrostatic considerations one can calculate the dipole-moment surface density corresponding to the measured change of the work function.<sup>3</sup> The work function change, ''??<sub>m''</sub>, caused by a sheet of dipoles residing on the surface is expressed by the Helmholtz equation:<sup>3,15</sup>		On the basis of simple electrostatic considerations one can calculate the dipole-moment surface density corresponding to the measured change of the work function.<sup>3</sup> The work function change, ''Δ φ<sub>m''</sub>, caused by a sheet of dipoles residing on the surface is expressed by the Helmholtz equation:<sup>3,15</sup>

	:<math>\Delta \phi_m = \frac {e \mu} {A \varepsilon \varepsilon_0}\,\!</math> Equation 3		:<math>\Delta \phi_m = \frac {e \mu} {A \varepsilon \varepsilon_0}\,\!</math> Equation 3

Cmditradmin at 19:47, 23 September 2009

2009-09-23T19:47:27Z

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	=== Work Function of Metals – Influence of Adsorbates ===		=== Work Function of Metals – Influence of Adsorbates ===
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	<br clear='all'>		<br clear='all'>
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	# '''''References'''''		'''''References'''''

	(1)Cahen, D.; Kahn, A. ''Adv. Mater.'' '''2003''', ''15'', 271-277.		(1)Cahen, D.; Kahn, A. ''Adv. Mater.'' '''2003''', ''15'', 271-277.

Cmditradmin: /* Work Function of Metals – Influence of Adsorbates */

2009-09-23T19:46:39Z

Work Function of Metals – Influence of Adsorbates

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	<sup><nowiki>*</nowiki></sup> The change in the work function, ''Δ φ<sub>m''</sub>,'' ''is defined as the difference in the work function of the clean substrate and the work function of the surface after the adsorption of inert gas atoms.		<sup><nowiki>*</nowiki></sup> The change in the work function, ''Δ φ<sub>m''</sub>,'' ''is defined as the difference in the work function of the clean substrate and the work function of the surface after the adsorption of inert gas atoms.


	#

	=== Self-Assembled Monolayers of Organic Thiols on Metals ===		=== Self-Assembled Monolayers of Organic Thiols on Metals ===

Cmditradmin: /* Work Function of Metals – Influence of Adsorbates */

2009-09-23T19:46:06Z

Work Function of Metals – Influence of Adsorbates

← Older revision		Revision as of 12:46, 23 September 2009
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	The work function of a metal is very sensitive to the presence of any adsorbates present on the surface. Experiments have revealed that even physisorbed atoms of inert gases such as argon or xenon influence the work function on a variety of metallic substrates.<sup>11-13</sup> Even though there is no significant charge redistribution in the inert-gas atoms near the metal surface, due to a chemical reaction, there is a substantial charge redistribution on the surface after the physisorption has taken place, which changes the surface dipole on the surface. Two major physical phenomena are responsible for this. First, the electronic density extending outside of the metal surface is pushed back by the repulsive force of the electronic cloud of the inert gas atoms; this is referred to as Pauli push-back, or sometimes as the “pillow effect”.<sup>3,14</sup> Second, van der Waals interactions between the inert-gas atom and the metal surface cause polarization of the otherwise highly symmetric electronic cloud of the inert-gas atom.~~<ref name="ftn5">'''~~ The two described mechanisms provide an oversimplified picture of the charge-redistribution process. Contribution from orbital interactions is also invoked as another mechanism of charge redistribution. See ref. 24.'''		The work function of a metal is very sensitive to the presence of any adsorbates present on the surface. Experiments have revealed that even physisorbed atoms of inert gases such as argon or xenon influence the work function on a variety of metallic substrates.<sup>11-13</sup> Even though there is no significant charge redistribution in the inert-gas atoms near the metal surface, due to a chemical reaction, there is a substantial charge redistribution on the surface after the physisorption has taken place, which changes the surface dipole on the surface. Two major physical phenomena are responsible for this. First, the electronic density extending outside of the metal surface is pushed back by the repulsive force of the electronic cloud of the inert gas atoms; this is referred to as Pauli push-back, or sometimes as the “pillow effect”.<sup>3,14</sup> Second, van der Waals interactions between the inert-gas atom and the metal surface cause polarization of the otherwise highly symmetric electronic cloud of the inert-gas atom.The two described mechanisms provide an oversimplified picture of the charge-redistribution process. Contribution from orbital interactions is also invoked as another mechanism of charge redistribution. See ref. 24.'''


	~~</ref>~~ This results in the formation of a sheet of dipoles in the space occupied by the adsorbed atoms which alters the potential landscape at the vacuum / metal interface.<sup>12,13'''</sup>		This results in the formation of a sheet of dipoles in the space occupied by the adsorbed atoms which alters the potential landscape at the vacuum / metal interface.<sup>12,13'''</sup>

	On the basis of simple electrostatic considerations one can calculate the dipole-moment surface density corresponding to the measured change of the work function.<sup>3</sup> The work function change, ''??<sub>m''</sub>, caused by a sheet of dipoles residing on the surface is expressed by the Helmholtz equation:<sup>3,15</sup>		On the basis of simple electrostatic considerations one can calculate the dipole-moment surface density corresponding to the measured change of the work function.<sup>3</sup> The work function change, ''??<sub>m''</sub>, caused by a sheet of dipoles residing on the surface is expressed by the Helmholtz equation:<sup>3,15</sup>

Cmditradmin: /* Self-Assembled Monolayers of Organic Thiols on Metals */

2009-09-23T19:45:34Z

Self-Assembled Monolayers of Organic Thiols on Metals

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	As discussed earlier, the work function of metals is affected by adsorbates present on the surface. The knowledge of adsorption characteristics of organic thiols on metals and the synthetic accessibility of a variety of structures makes these SAMs excellent systems to be employed in the systematic study of the influence of adsorbates on the work function of the metallic substrates.		As discussed earlier, the work function of metals is affected by adsorbates present on the surface. The knowledge of adsorption characteristics of organic thiols on metals and the synthetic accessibility of a variety of structures makes these SAMs excellent systems to be employed in the systematic study of the influence of adsorbates on the work function of the metallic substrates.


	#

	=== Self-Assembled Monolayers of Alkanethiols and their Influence on the Work Function of Metals ===		=== Self-Assembled Monolayers of Alkanethiols and their Influence on the Work Function of Metals ===

Cmditradmin: /* Work Function of Metals – Influence of Adsorbates */

2009-09-23T19:45:18Z

Work Function of Metals – Influence of Adsorbates

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	=== Work Function of Metals – Influence of Adsorbates ===		=== Work Function of Metals – Influence of Adsorbates ===
	~~'''''~~

	The work function of a metal is very sensitive to the presence of any adsorbates present on the surface. Experiments have revealed that even physisorbed atoms of inert gases such as argon or xenon influence the work function on a variety of metallic substrates.<sup>11-13</sup> Even though there is no significant charge redistribution in the inert-gas atoms near the metal surface, due to a chemical reaction, there is a substantial charge redistribution on the surface after the physisorption has taken place, which changes the surface dipole on the surface. Two major physical phenomena are responsible for this. First, the electronic density extending outside of the metal surface is pushed back by the repulsive force of the electronic cloud of the inert gas atoms; this is referred to as Pauli push-back, or sometimes as the “pillow effect”.<sup>3,14</sup> Second, van der Waals interactions between the inert-gas atom and the metal surface cause polarization of the otherwise highly symmetric electronic cloud of the inert-gas atom.<ref name="ftn5">''' The two described mechanisms provide an oversimplified picture of the charge-redistribution process. Contribution from orbital interactions is also invoked as another mechanism of charge redistribution. See ref. 24.'''		The work function of a metal is very sensitive to the presence of any adsorbates present on the surface. Experiments have revealed that even physisorbed atoms of inert gases such as argon or xenon influence the work function on a variety of metallic substrates.<sup>11-13</sup> Even though there is no significant charge redistribution in the inert-gas atoms near the metal surface, due to a chemical reaction, there is a substantial charge redistribution on the surface after the physisorption has taken place, which changes the surface dipole on the surface. Two major physical phenomena are responsible for this. First, the electronic density extending outside of the metal surface is pushed back by the repulsive force of the electronic cloud of the inert gas atoms; this is referred to as Pauli push-back, or sometimes as the “pillow effect”.<sup>3,14</sup> Second, van der Waals interactions between the inert-gas atom and the metal surface cause polarization of the otherwise highly symmetric electronic cloud of the inert-gas atom.<ref name="ftn5">''' The two described mechanisms provide an oversimplified picture of the charge-redistribution process. Contribution from orbital interactions is also invoked as another mechanism of charge redistribution. See ref. 24.'''

Cmditradmin: /* Work Function of Metals – Implications for Organic Electronics Applications */

2009-09-23T19:45:06Z

Work Function of Metals – Implications for Organic Electronics Applications

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	[[Image:energy_ecl.jpg\|thumb\|300px\|Figure 3. Energy diagram of an organic electroluminescent device. The charge carriers – electrons (e-) and holes (h+) – are injected respectively from the metal cathode (right-hand side of the diagram) and the anode (left-hand side of the diagram) into the organic active layers – the electron-transport layer (ETL) and the hole-transport layer (HTL). At the ETL / HTL interface the charge carriers recombine generating photons. The Fermi levels of both the metal cathode and the anode and the energy levels of the organic layers (HOMO and LUMO energy levels) have to be matched in order to achieve an efficient charge injection into the organic layers. Based on ref. 2. ]]		[[Image:energy_ecl.jpg\|thumb\|300px\|Figure 3. Energy diagram of an organic electroluminescent device. The charge carriers – electrons (e-) and holes (h+) – are injected respectively from the metal cathode (right-hand side of the diagram) and the anode (left-hand side of the diagram) into the organic active layers – the electron-transport layer (ETL) and the hole-transport layer (HTL). At the ETL / HTL interface the charge carriers recombine generating photons. The Fermi levels of both the metal cathode and the anode and the energy levels of the organic layers (HOMO and LUMO energy levels) have to be matched in order to achieve an efficient charge injection into the organic layers. Based on ref. 2. ]]
	~~[[Image:]]~~

	'''		'''

Cmditradmin: /* Self-Assembled Monolayers of Conjugated Thiols and their Influence on the Work Function of Metals */

2009-09-23T19:43:06Z

Self-Assembled Monolayers of Conjugated Thiols and their Influence on the Work Function of Metals

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	#		#

			<br clear='all'>
	=== Self-Assembled Monolayers of Conjugated Thiols and their Influence on the Work Function of Metals ===		=== Self-Assembled Monolayers of Conjugated Thiols and their Influence on the Work Function of Metals ===
	~~'''''~~

	A limitation on the use of monolayers consisting of long-chain alkyl thiols for modifying injection barriers between metals and organics is the intrinsic electrically insulating characteristics of the alkyl chains. Monolayers consisting of ?-conjugated thiols, which have been shown to exhibit considerably enhanced conductivity,<sup>26,35</sup> may be more promising for this type of application. Monolayers composed of ?-conjugated systems based on oligo(phenylethynyl)benzenethiols and oligo(phenyl)benzenethiols have been shown to form self-assembled domains on gold.<sup>21,22,36-38</sup> A variety of data from different measurement techniques supports the formation of these SAMs on gold and silver with the thiol group binding to the underlying metal and the molecular backbone orienting itself nearly perpendicularly to the surface plane of the substrate.<sup>21,22,36,39-41</sup> Even in the case of molecular structures based on biphenyl thiols with very polar substituents this structural arrangement seems to hold, as demonstrated by Kang et al. in their comprehensive report.<sup>22</sup> As with alkylthiolate SAMs, the understanding of the structure of ?-conjugated thiolate SAMs on metal substrates opened the possibility to study the influence of the molecular structure on the electronic properties of the underlying substrates.
			A limitation on the use of monolayers consisting of long-chain alkyl thiols for modifying injection barriers between metals and organics is the intrinsic electrically insulating characteristics of the alkyl chains. Monolayers consisting of ?-conjugated thiols, which have been shown to exhibit considerably enhanced conductivity,<sup>26,35</sup> may be more promising for this type of application. Monolayers composed of π-conjugated systems based on oligo(phenylethynyl)benzenethiols and oligo(phenyl)benzenethiols have been shown to form self-assembled domains on gold.<sup>21,22,36-38</sup> A variety of data from different measurement techniques supports the formation of these SAMs on gold and silver with the thiol group binding to the underlying metal and the molecular backbone orienting itself nearly perpendicularly to the surface plane of the substrate.<sup>21,22,36,39-41</sup> Even in the case of molecular structures based on biphenyl thiols with very polar substituents this structural arrangement seems to hold, as demonstrated by Kang et al. in their comprehensive report.<sup>22</sup> As with alkylthiolate SAMs, the understanding of the structure of ?-conjugated thiolate SAMs on metal substrates opened the possibility to study the influence of the molecular structure on the electronic properties of the underlying substrates.

	Campbell et al. studied the effects of oligo(phenylethynyl)benzenethiolate SAMs on copper on the performance of organic diodes.<sup>42</sup> Two molecular systems were compared, with the substitution of a terminal hydrogen atom for a fluorine atom as the only difference between the SAM constituents. The researchers compared the current – voltage characteristics of organic diodes with different structures – Cu anode / MEH-PPV / Ca, and Cu anode / SAM / MEH-PPV / Ca. The onset voltage for the conduction in the studied devices showed the expected trend, with the lowest onset recorded for the SAM composed of the fluorine-substituted molecules and the highest onset voltage for the corresponding SAM without fluorine substitution. The rationalization of the observed effect invoked the difference in the hole-injection barrier from the electrode to the organic layer due to the change in the work function caused by the formation of SAMs. Kelvin probe measurements indeed showed that the work function of the copper electrodes coated with the different SAMs qualitatively follows the prediction based on the well known effect of the sheet of dipole moments on top of the metal.		Campbell et al. studied the effects of oligo(phenylethynyl)benzenethiolate SAMs on copper on the performance of organic diodes.<sup>42</sup> Two molecular systems were compared, with the substitution of a terminal hydrogen atom for a fluorine atom as the only difference between the SAM constituents. The researchers compared the current – voltage characteristics of organic diodes with different structures – Cu anode / MEH-PPV / Ca, and Cu anode / SAM / MEH-PPV / Ca. The onset voltage for the conduction in the studied devices showed the expected trend, with the lowest onset recorded for the SAM composed of the fluorine-substituted molecules and the highest onset voltage for the corresponding SAM without fluorine substitution. The rationalization of the observed effect invoked the difference in the hole-injection barrier from the electrode to the organic layer due to the change in the work function caused by the formation of SAMs. Kelvin probe measurements indeed showed that the work function of the copper electrodes coated with the different SAMs qualitatively follows the prediction based on the well known effect of the sheet of dipole moments on top of the metal.
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	Zehner et al. were the first to systematically study the work function modification of gold by conjugated thiols.<sup>43</sup> The authors applied Kelvin-probe measurements to a series of oligo(phenylethynyl)benzenethiolate SAMs on gold. The rather impressive number of studied systems included molecular structures with different lengths of the ?-conjugated backbones as well as different polar end-substituents. The trends of the work-function change reported in this work are actually not consistent with the predictions based on the molecular dipole moment. In particular, the molecular systems with dipole moments that should theoretically lower the work function actually increase it. Nevertheless, the authors attributed the observed work-function changes to the different dipole moment values of the studied molecular systems, and further conclusions followed.		Zehner et al. were the first to systematically study the work function modification of gold by conjugated thiols.<sup>43</sup> The authors applied Kelvin-probe measurements to a series of oligo(phenylethynyl)benzenethiolate SAMs on gold. The rather impressive number of studied systems included molecular structures with different lengths of the ?-conjugated backbones as well as different polar end-substituents. The trends of the work-function change reported in this work are actually not consistent with the predictions based on the molecular dipole moment. In particular, the molecular systems with dipole moments that should theoretically lower the work function actually increase it. Nevertheless, the authors attributed the observed work-function changes to the different dipole moment values of the studied molecular systems, and further conclusions followed.

	Chen et al. investigated work function of gold substrates coated with a series of differently substituted terphenyl thiolate SAMs.<sup>44</sup> All of the studied systems showed a depression of the work function when compared with clean gold. A gold surface with an overlayer of a SAM of terphenylthiolate, characterized by only a small dipole moment, showed a work-function value 0.90 eV lower than that measured for the clean gold. Because the dipole moment of the molecule is small, this change has to be attributed to the charge redistribution caused by the formation of Au – S bond. Substitution of hydrogen on phenyl rings in the terphenylbackbone with fluorine atoms showed an increase of the work function of the underlying substrate, in accordance with qualitative predictions based on the dipole moment considerations. The authors showed their ability to tune the work function from 4.30 eV to 4.95 eV by simply using different SAMs on top of the gold electrodes. The structures of the SAMs and the corresponding values of the measured work function are shown in Figure 9. Further, the authors showed changes in the hole-injection barrier, ?<sub>h</sub>, into a copper-phthalocyanine (CuPc) layer deposited on top of the different SAM-coated gold substrates. This was done by measuring the position of the highest molecular energy level of CuPc (HOMO) with respect to the Fermi level for the different samples. The changes in the ''measured'' hole-injection barrier roughly follow the dependence: ''?<sub>h </sub>~~? ?~~<sub>m''</sub>, where ''?<sub>m''</sub> is the work function ''measured'' for the SAM-coated gold substrates. Thus, the use of the different ?-conjugated-thiolate SAMs allowed for tuning the hole-injection barrier from the gold electrode into CuPc organic layer. The values of the hole-injection barrier, ''?<sub>h''</sub>, are also shown in Figure 9.		Chen et al. investigated work function of gold substrates coated with a series of differently substituted terphenyl thiolate SAMs.<sup>44</sup> All of the studied systems showed a depression of the work function when compared with clean gold. A gold surface with an overlayer of a SAM of terphenylthiolate, characterized by only a small dipole moment, showed a work-function value 0.90 eV lower than that measured for the clean gold. Because the dipole moment of the molecule is small, this change has to be attributed to the charge redistribution caused by the formation of Au – S bond. Substitution of hydrogen on phenyl rings in the terphenylbackbone with fluorine atoms showed an increase of the work function of the underlying substrate, in accordance with qualitative predictions based on the dipole moment considerations. The authors showed their ability to tune the work function from 4.30 eV to 4.95 eV by simply using different SAMs on top of the gold electrodes. The structures of the SAMs and the corresponding values of the measured work function are shown in Figure 9. Further, the authors showed changes in the hole-injection barrier, Δ<sub>h</sub>, into a copper-phthalocyanine (CuPc) layer deposited on top of the different SAM-coated gold substrates. This was done by measuring the position of the highest molecular energy level of CuPc (HOMO) with respect to the Fermi level for the different samples. The changes in the ''measured'' hole-injection barrier roughly follow the dependence: ''Δ<sub>h </sub>&prop; φ<sub>m''</sub>, where ''φ<sub>m''</sub> is the work function ''measured'' for the SAM-coated gold substrates. Thus, the use of the different π-conjugated-thiolate SAMs allowed for tuning the hole-injection barrier from the gold electrode into CuPc organic layer. The values of the hole-injection barrier, ''Δ<sub>h''</sub>, are also shown in Figure 9.

	~~[[Image:]]~~

	'''Figure 9. Schematic representation of SAMs on gold studied by Chen et al. The measured work functions for the SAM-coated substrates, ?<sub>m</sub>, as well as the hole-injection barriers measured for Au / SAM / CuPc systems, ?<sub>h</sub>, are shown. Based on ref. 44. '''

			[[Image:workfunctions_sam.jpg\|thumb\|300px\|'''Figure 9. Schematic representation of SAMs on gold studied by Chen et al. The measured work functions for the SAM-coated substrates, φ<sub>m</sub>, as well as the hole-injection barriers measured for Au / SAM / CuPc systems, ?<sub>h</sub>, are shown. Based on ref. 44. ''']]

	Apart from other experimental reports on the metal-work-function changes induced by ?-conjugated SAMs,<sup>45,46</sup> particular attention should be brought to theoretical work dealing with the energy level alignment of organic overlayers on gold. Among those, the reports by Heimel et al.,<sup>3,47</sup> and Romaner et al.<sup>15,48</sup> are perhaps of the highest interest in the context of this thesis. These studies focus on theoretical description of biphenylthiolate-based systems as there is a vast amount of data showing these SAMs possess two dimensional order and their structure has been studied extensively, thus making them good model systems for theoretical investigations.<sup>19,22</sup> Employing density functional theory (DFT), Heimel et al. showed that biphenylthiolate SAMs terminated with three different end-groups (a weak ?-donor –SH, a strong ?-donor –NH<sub>2</sub>, and a strong ?-acceptor –CN; see Figure 10) can result in large changes of the work function of the underlying gold substrate.<sup>3,47</sup>		Apart from other experimental reports on the metal-work-function changes induced by ?-conjugated SAMs,<sup>45,46</sup> particular attention should be brought to theoretical work dealing with the energy level alignment of organic overlayers on gold. Among those, the reports by Heimel et al.,<sup>3,47</sup> and Romaner et al.<sup>15,48</sup> are perhaps of the highest interest in the context of this thesis. These studies focus on theoretical description of biphenylthiolate-based systems as there is a vast amount of data showing these SAMs possess two dimensional order and their structure has been studied extensively, thus making them good model systems for theoretical investigations.<sup>19,22</sup> Employing density functional theory (DFT), Heimel et al. showed that biphenylthiolate SAMs terminated with three different end-groups (a weak ?-donor –SH, a strong ?-donor –NH<sub>2</sub>, and a strong ?-acceptor –CN; see Figure 10) can result in large changes of the work function of the underlying gold substrate.<sup>3,47</sup>

	[[Image:]]		[[Image:bpt_sam_wf.jpg\|thumb\|400px\|'''Figure 10. Schematic representation of biphenylthiolate SAMs on gold studied theoretically by Heimel et al. The calculated changes of the work function caused by the SAMs, Δ φ<sub>m</sub>, and the offsets between the HOMO and the Fermi level, Δ<sub>HOMO</sub>, are shown. Results from ref. 47. '''
			]]
	'''Figure 10. Schematic representation of biphenylthiolate SAMs on gold studied theoretically by Heimel et al. The calculated changes of the work function caused by the SAMs, ??<sub>m</sub>, and the offsets between the HOMO and the Fermi level, ?<sub>HOMO</sub>, are shown. Results from ref. 47. '''