Room Temperature Negative Differential Resistance in Nanoscale Molecular
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Molecular devices utilizing active self-assembled monolayer (SAM) (containing nitroamine (2'-amino-4-ethynylphenyl-4'-ethynylphenyl-5'-nitro-1-benzenethiolate) and nitro (4-ethynylphenyl-4'-ethynylphenyl-2'-nitro -1-benzenethiolate) redox center) as the active component are reported. Current-voltage measurements of the devices exhibited negative differential resistance at room temperature and an on-off peak-to-valley ratio in excess of 1000:1 at low temperature. The discovery of negative differential resistance (NDR) in semicoductor diodes opened a new chapter in semiconductor device physics [1]. The physical basis of the Esaki diode is interband tunneling between the valence band and the conduction band. NDR can also result from resonant tunneling in semiconductor heterostructures [2]. The presence of NDR at room temperature allows for many practical applications [3, 4, 5]. Here we report the observation of large NDR behavior, and room temperature operation, in an electronic device that utilizes molecules as the active component. Electronic measurements were performed in a nanostructure consisting of top metal contact (Au) - self-assembled monolayer (SAM) active region (30 to 50 nm in D A Si
Si
SiO 2
Si3N4 B Si3N4 Au C u Au O2 N
Au E
O2 N NH2 S
O2 N NH2 S
Au
NH2 S
100nm
Figure 1. Schematics of device fabrication: (A ) cross section of a silicon wafer with a nanopore etched through a suspended silicon nitride membrane; (B) Au-SAM-Au junction in the pore area; (C) blowup of (B) with 1 sandwiched in the junction; (D) scanning electron micrograph (SEM ) of pyramid Si structure after unisotropic Si etching (that is, the bottom view of (A )); (E) SEM of an etched nanopore through the silicon nitride membrane. diameter) - bottom metal contact (Au), similar to that reported previously [6, 7]. The active electronic component was made from 2'-amino-4-ethynylphenyl-4'-ethynylphenyl5'-nitro -1-(thioacetyl)benzene (1a) that was prepared as outlined in Figure 1. A
NHAc
N H2 Br
Br
N H Ac
H
1. Ac 2 O, 88% Br 2. HNO3 H2 SO4 69% O2 N
Br
Br
Pd(PPh3 ) 2Cl 2 , PPh3 CuI, NEt3 , 42%
3
4
O 2N
N H2 1. HCl (3M), THF, 100% Z 2. A c S
H
O2 N
Pd(PPh3) 2 Cl 2 , PPh3 CuI, NEt3 , 67%
N H4 O H
1a, Z = SCOCH3 1b, Z = SH 1 , Z = S¯
NO 2
NO 2
B
TMS Br
Br
H Br
Pd/Cu
TMS
6 NO 2
H
6
TMS
Pd/Cu 26.4%
7
7
1)K2CO3 MeOH/CH2Cl2
NO2 SAc
2) I
SAc Pd/Cu
9
19.1%
Figure 2. (A ) Schematic of the synthesis of the active molecular compound and its precursors (1a-b and 1); (B) schematic of the synthesis of the active molecular compound 2. Acylation and nitration of 2,5-dibromoaniline afforded 3 [8] which underwent Pd/Cucatalyzed coupling [9] with phenylacetylene preferentially at the more electrophilic C-Br site to yield 4. Acetate hydrolysis and coupling with 4-ethynyl(thioacetyl)benzene [10] afforded the desired compound 1a. A s we established previously, thioacetyl groups can be selectively hydrolyzed with ammonium hydroxide in tetrahydrofuran (THF) during the self-assembly step to afford the free thiol, 2'-amino-
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