Page 10 - Molecules for Charge-Based Information Storage

Molecules for Charge-Based Information Storage Lindsey and Bocian

FIGURE 10. Increase in charge density accompanying stepwise synthesis (left panel). AFM examination of surface to assess stepwise synthesis of
charge-storage molecules (right panel).

synthesis afford multiple molecular layers that build verti- attachment (step 1), a dianhydride (BPTC) reacts with the
cally off the surface, whereas dyads and triads are incorpo- amine to give a monoimide, leaving one anhydride free
rated as monolayers. The use of the vertical dimension (step 2). In step 3, a diamino-porphyrin (2) is attached to
affords higher charge density than a monolayer and pro- the anhydride, forming the imide and leaving the distal
vides a more robust surface for deposition of a top contact. amine free. Repetition of steps 2 and 3 allows successive
Upon examination of diverse porphyrins for attachment layers of porphyrins to be added to the growing chain. This
via carbon-terminated groups to Si, porphyrins bearing two on-chip assembly is compatible with semiconductor fabrica-
or more ethynes were found to afford polymers. 34 Films tion requirements.
have been prepared on Si(100), SiO 2 , Au(111), and glass. The The stepwise growth process was readily observed by
thickness of the resulting polymer could be controlled by the FTIR spectroscopy, XPS, AFM, and electrochemistry. FTIR
amount of porphyrin and the duration of polymerization. spectroscopy enabled monitoring of the presence/disap-
SEM analysis indicates the porphyrin polymer films range in pearance of the anhydride. The increase in charge density
thickness from tens to hundreds of nanometers with stepwise synthesis and assessment of the stepwise
(Figure 8ac). The polymers contain intact porphyrin macro- growth process by interrogation with AFM are shown in
cycles and are redox-active albeit with quite slow electron- Figure 10.
transfer rates. Regardless, this serendipitous finding pro- The incorporation of molecules in electronic devices
vided one solution to the challenge of creating redox-active requires making hybrid junctions wherein molecules are
materials with high charge density. sandwiched between two metal contacts or a metal and a
To exercise molecular-level control over size and compo- semiconductor contact. However, the fate of molecules
sition, methods for stepwise syntheses 41,42 or patterning subsequent to deposition of a top metal contact has gen-
(i.e., selective derivatization) 27,41,46 of charge-storage mole- erally not been well characterized. Toward this goal, the
cules on an electroactive surface were developed. One interaction of evaporated Cu, Ag, and Au films deposited in
strategy developed, while inspired by solid-phase synthesis varying thicknesses (3, 5, and 8 nm) on a series of mono-
of biomolecules, employs no protecting groups (Figure 9). 41,42 layer-coverage porphyrins covalently attached to Si(100)
The first porphyrin (1) bears a tether for attachment to Si substrates was investigated. 46,48,49 The methods of inter-
(in this case, an all-carbon tripod) and a distal amine. After rogation included ellipsometry, AFM, FTIR spectroscopy,

Vol. 44, No. 8 ’ 2011 ’ 638–650 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 647