Cu TCNQ

Heintz RA, Zhao H, Ouyang X, Grandinetti G, Cowen J, Dunbar KR (1999) New insight into the nature of Cu(TCNQ) solution routes to two distinct polymorphs and their relationship to crystalline films that display bistable switching behavior. Inorg Chem 38 144—156... 

Infrared Reflectance Spectra of Cu-TCNQ Semiconducting Films... 

To investigate the formal charge of TCNQ in the semiconducting films of Cu-TCNQ, the infrared reflectance spectra was recorded at room temperature for crystalline Cu-TCNQ films before and after an external electric field was applied to the sample. 

The Cu-TCNQ switching material was subjected to electric fields by clamping a thin highly insulating film of either teflon or polyethylene between the surface of the Cu-TCNQ film on a cop-... 

In Table I, the results of this experiment are compared to reflectance spectra measured for other simple and complex metal-TCNQ salts. We found that the CN stretching mode in reflectance measurements shifted to higher frequency by about 100 cm from absorption measurements made on the same material. The peak in the reflectance band at 2320 cm for the Cu-TCNQ film prior to the application of a field is consistent with the values measured for the simple (1 1) salts of Li+(TCNQ ) and Cu+(TCNQ") tabulated in Table I. These crystalline materials are simple salts which do not contain neutral TCNQ . On the other hand the spectra of a Cu-TCNQ film after the application of an applied field closely resembles the spectra of Cs2 (TCNQ [ )3 with two CN stretching modes separated by 20 cm. Cs2(TCNQ )3 is a complex salt which contains neutral TCNQ and radical-anion TCNQ . (11)... 

The diffuse reflectance spectra reported in Table I show that it is possible to assign a CN stretching frequency to both neutral and radical-anion TCNQ in crystalline samples of metal-TCNQ complexes because the reflectance peak for neutral TCNQ is shifted 20 cm higher in frequency than for radical-anion TCNQ". Specifically, the reflectance data for Cu-TCNQ when compared to other metal-TCNQ salts of known composition strongly suggests that neutral TCNQ is not present in the unswitched Cu-TCNQ films. On the other hand, the additional peak that appears in the spectra of Cu-TCNQ subjected to an applied field shows a peak superimpos-able with the peak recorded for neutral TCNQ in Cs2(TCNQ" )3. 

This evidence suggests that neutral TCNQ is formed in a solid-state field induced phase transition when electric fields are applied to crystalline films of Cu-TCNQ grown on copper substrates. 

Figure 6. Reflectance spectra of a crystalline film of Cu-TCNQ on copper before and after the application of an electric field.

Cu TCNQ SIMPLE 1 1 SALT PREPARED BY METATHETICAL REACTIQN 2323... 

Cu TCNQ SWITCH BEFQRE APPLICATIQN QF ELECTRIC FIELD 2320... 

Cu TCNQ SWITCH AFTER APPLICATIQN QF ELECTRIC FIELD 2321 AND 2340... 

It Is postulated that mixed-valence species or complex salts (12) formed as a result of this field Induced redox reaction control the semiconducting behavior of these films and these complex salts exist In a solid-state equilibrium with the simple 1 1 salt. Since non-integral oxidation states are common In solids, It Is difficult to predict exact stoichiometry In the equilibrium equation, but a likely equation for switching In Cu-TCNQ, for example, may Involve... 

In the case of a mixed-valence salt containing neutral TCNQ there are more TCNQ molecules than there are unpaired electrons and, therefore, electrostatic repulsion of charge carriers Is kept at a minimum by allowing conduction electrons to occupy the empty molecular orbitals of TCNQ . This Is a lower energy pathway compared to putting more than one electron on the seime TCNQ site and It may explain how mixed-valence semiconducting salts like CS2 (TCNQ )s and the "switched" form of Cu-TCNQ can exhibit greater conductivity than similar salts with 1 1 stoichiometry. 

Charge transfer complexes have also been investigated as the most attractive candidate materials for high-density electrical data storage.76,77 For instance, sil-ver-tetracyanoquinodimethane (Ag-TCNQ) and copper-tetracyanoquinodimethane (Cu-TCNQ) have been studied for data recording since they exhibit electrical bistability. 

Upon application of a suitable bias voltage, the resistance of the stacked Cu/Cu(TCNQ)/Al device decreased abruptly and remained in a highly conducting state. The device could be switched back into its high resistance state when an opposite bias was applied. 

The following discussion is concerned with the influence of various contacts to Cu(TCNQ) and different device structures on the switching properties. As the switching process is activated electrically, an influence of the contact is... 

Figure 27.1 Typical I-V characteristics of a Cu/Cu(TCNQ)/Al device with two stable resistance states. Numbers indicate the voltage sweep sequence.

Further, we studied and traced the localisation of the switching in the device. Possible regions for switehing could be the interfaces of the metal contact to the Cu(TCNQ) or in the Cu(TCNQ) bulk. In order to localise the switching we ehanged the staeked strueture into a planar structure and extended it with an additional eleetrode. 

First, we analysed the influence of the top metal contact size on the switehing behaviour. We used Au as the contact metal to exclude the possible influenee of an unintentionally formed oxide interlayer between the metal and the Cu(TCNQ). 

Finally, the Au top contaet was substituted by a sharp Au tip and plaeed onto the Cu(TCNQ) layer (see Figure 27.5 for I-V characteristies). In this ease, pronounced switching occurred reproducibly continuously for many hundred sweep cycles. These results show that Cu(TCNQ) can be switched between two conductivity states with electrodes that do not form an oxide on their surface when the contact area is small enough. 

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