Ivo A.
Hümmelgen
*a,
Neil J.
Coville
b,
Isidro
Cruz-Cruz
a and
Rafael
Rodrigues
a
aDepartamento de Física, Universidade Federal do Paraná, Caixa Postal 19044, 81531-980 Curitiba, PR, Brazil. E-mail: iah@fisica.ufpr.br; Tel: +55-41-33613645
bDST-NRF Centre of Excellence in Strong Materials and Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, PO Wits 2050, Johannesburg, South Africa
First published on 16th June 2014
We briefly review recent developments concerning the use of carbon nanostructures in polymer composite thin films for write-once-read-many-times memory devices. We also show that carbon sphere/poly(vinylphenol) composites prepared with the addition of hexadecyltrimethylammonium bromide as a surfactant show better carbon sphere dispersion, allowing in principle a memory device size reduction. Devices constructed with surfactant added carbon-sphere/poly(vinylphenol) composites allow writing the ON state in ∼200 ns and the write operation consumes 5 × 10−5 J cm−2 when a pulse with an amplitude of 5 V and a length of 1 μs is used.
The development of solid state semiconductor-based electronics has helped promote the enormous technological progress that has taken place since the mid-20th century. Indeed, semiconductor memory devices have also experienced a huge performance improvement resulting in the widespread application of these devices.
In general, memory devices possess a set of characteristics (speed, data storage density, energy consumption, production and maintenance costs, capability to erase and rewrite information) that may make them very suitable for application in specific niche areas.
Despite earlier efforts,1,2 organic materials have only been intensively investigated for device applications in the last two decades of the 20th century. This revival of interest was due in part to the successful demonstration of organic materials in organic solar cells,3 light-emitting diodes,4 transistors5 and memory devices6–8.
The efforts to develop organic memory devices with real practical application potential9–11 led to the investigation of a large number of organic and hybrid systems in which resistive switching could be observed. This switching, the transition of a device from a high to a low resistance state (or vice versa), can be initiated by different physical (electronic or thermal) or chemical (redox or chemical degradation) principles. The essential feature required is a controlled transition between two states (OFF and ON, to which binary “0” and “1” codes can be assigned).
This condition can be obtained using a layer of carbon nanostructures embedded in a polymer matrix sandwiched between two conducting electrodes (Fig. 1). This simple structure is attractive due to the low potential production costs associated with most carbon nanostructures.
Methods to prepare the various shaped carbon nanostructures typically involve a bottom up approach that starts from small readily available molecules (CO, CH4, CH2CH2, C6H6etc.) that decompose at high temperature (typically between 400 and 1000 °C) to give fragments that combine to give the carbon materials.18 The decomposition usually takes place in the presence of a catalyst and it is the catalyst (or a template) that determines the shape/size of the final structure. In many carbon materials the catalyst used to make the carbons is difficult to remove and this contaminant can impact on the final electronic properties of the carbon. CSs however are usually made in the absence of a catalyst.19 The decomposition process that is generally employed to produce the carbons in reasonable yields can be achieved in a batch process (in autoclaves) or a flow system (e.g. chemical vapor deposition (CVD)). The final product morphology and yields are also determined by the reaction parameters (gas flow/pressure/temperature). The mechanism by which the structures form has been discussed in numerous reviews and the latest views on this topic can be found in ref. 20.
Once made the carbons are hydrophobic and are difficult to process further. To allow processing the carbons are typically functionalized by oxidation, or reacted with acids or other chemical reagents, and this generates hydrophilic surface groups that aid their ‘solubility’ in solvents.21 Thus, if a carbon is to be mixed with, for example, a polymer, then some surface modification is required. The external surface properties are also modified by this process. An alternative to the above is to dope the carbon materials by replacing carbon atoms with N or B atoms, either via in situ procedures or by post-doping the carbons.22
In the operation of such devices, the read voltage is selected in the range where the two states are clearly distinguishable (0 < Vread < Vth; see Fig. 2(a)). When increasing voltage is applied to a device, it initially allows a low current IOFF to pass (Fig. 2(b)). However, after applying a write voltage pulse, the current at read voltage is significantly higher, equal to ION. If the write operation is complete, the system will not return to its previous condition, defining the write-once-read-many (WORM) characteristic of the device, as will be discussed later.
Carbon structurea | Matrixb | Conc. (w/w%) | I ON/IOFFc | Ref. |
---|---|---|---|---|
a Vulcan XC-72J: commercial material; CS: carbon sphere; N-CS: nitrogen modified carbon sphere; B-CS: boron modified carbon sphere; f-CNS: functionalized carbon nanoshell; RGO: reduced graphene oxide. b PAN: polyaniline, PVK: poly(N-vinyl carbazole), PVP: poly(vinylphenol). c The ON/OFF current ratio was calculated at V = 1 V, except for the Vulcan XC-72 case, where it was measured at V = 0.5 V. | ||||
CNT | PAN | >1 | 105 | 27 |
CNT | PVK | 1 | 103 | 28 |
CNT | PVP | 0.10 | 106 | 29 |
N-CNT | PVP | 0.20 | 105 | 29 |
B-CNT | PVP | 0.10 | 106 | 29 |
Vulcan XC-72J | PVP | 106 | 24 | |
CS | PVP | 10 | 106 | 23 |
N-CS | PVP | 10 | 106 | 25 |
B-CS | PVP | 10 | 105 | 25 |
f-CNS | PVP | 10 | 106 | 26 |
RGO | PVP | 0.50 | 105 | 30 |
Among the composites used to make devices, those containing carbon nanostructures (with different shapes), and poly(vinylphenol) (PVP) as the polymer matrix, have shown the highest ON/OFF current ratio (ca. 106; see Table 1). Despite the fact that the distributions of ION and IOFF values are typically rather broad for these composites, the ‘eye-opening’ still encompasses four orders of current magnitude25 allowing easy electronic distinction between the two states.
However, tt is only a measure of the time required to establish the highly conducting connection. If the voltage pulse is stopped immediately after the occurrence of the OFF to ON transition, followed by the application of Vread, the ON state current is not maintained at the level expected from the measured I(V) characteristics as shown in Fig. 2(a). The current in this case presents lower values, partially restoring the OFF condition. The irreversibility of the transition is only achieved when longer pulses of Vwrite amplitude are applied to consolidate the ON state condition. This so-called consolidation time (tc) is longer than tt, the exact value depending on the particular carbon structure. For the case of undoped carbon spheres this is of the order of 10 μs.25 The consolidation time can be estimated by applying pulse trains of amplitude Vwrite followed by Vread between pulses under the condition that the time between pulses is much longer than the pulse length and pulse duration, tp. If tp is long enough to consolidate the ON state, the ION value is achieved immediately after the first pulse, remaining the same after any following pulses. However, if during the write tp < tc, the current value (after the pulses) progressively grows after each pulse, only achieving the stationary ION value after numerous pulses that depend on the tp/tc ratio.25
A study of PVP–carbon sphere (undoped, N-doped and B-doped) composites revealed that the consolidation time was shorter for the spheres that showed the largest mass loss at low temperatures as indicated by thermogravimetric experiments. This suggested that the “welding” of carbon structures that leads to the retention of the low resistance ON condition correlates with the less stable carbon structures.
When carbon spheres are functionalized by refluxing in concentrated nitric acid the agglomeration tendency is reduced, but not totally suppressed.26 Good dispersion, with an almost total suppression of agglomerates, was observed when functionalized nanoshells31 were applied in memory devices.26
A more general procedure that would allow the use of carbon nanostructures without functionalization and that is applicable to nanostructures of different sizes and shapes would be highly desirable. In this sense, an interesting approach was developed by Dölle et al.32 for the dispersion of carbon nanotubes. This was later also applied to the dispersion of non-functionalized carbon spheres in water33 to prepare composites in a poly(vinylalcohol) matrix for pressure sensor applications. The procedure essentially uses hexadecyltrimethylammonium bromide (CTAB), a surfactant, which is mixed with the carbon nanostructures in water and provides for better water–carbon interactions. The hydrophilic extremity of the CTAB molecule provides miscibility in water, whereas the hydrophobic extremity tends to attach to the carbon structure. When a mixture of surfactant solution and carbon nanostructures is cooled below the Krafft temperature (TK = 25 °C for CTAB32) the micelle formation is suppressed and only a fraction of the surfactant stays in solution as determined by the critical micelle concentration (∼0.036 wt% for CTAB). The excess of surfactant precipitates in the form of hydrated crystalline needles and can be separated. The addition of the carbon nanostructures and further ultrasonication allow the separation of carbon structures that are covered by the dissolved surfactant from the solution. Thus, when the solution is left to stand at T < TK, the excess of surfactant sediments out and the supernatant is a solution that contains mostly individually dispersed carbon structures and a minimum amount of surfactant that is required for stabilizing the nanoparticles.32
The composite is prepared by mixing the two components (solution 1 and suspension 2) together. Solution 1 is prepared by mixing PVP and methylated poly(melamine-co-formaldehyde) (PMF), as a cross-linker, dissolved in propylene glycol monomethyl ether acetate at 7 wt%. The mixture is left for 1 h at room temperature in ultrasound to dissolve the PVP. Suspension 2 is prepared by adding CTAB (10 mg mL−1) to PMF, followed by the addition of CSs (10 wt%). The mixture is then sonicated for ca. 30 min at room temperature and then for ca. 1 h at 0 °C. Suspension 2 is left to stand at 5 °C (below the CTAB TK) for 4 days and then the supernatant (ca. 50%) is taken and mixed with solution 1 at a volume proportion of 1:
5 to give the final suspension. In each case the suspension (100 μL) is spin-coated at 2000 rpm onto an Al bottom electrode. The Al electrode was evaporated onto a 10 × 25 mm2 glass substrate at a base pressure of 10−6 torr, and patterned using a shadow mask. The devices are annealed in air at 200 °C for an hour in order to cross-link the polymer.37–39 Au is then evaporated as a top electrode under the same conditions as for the bottom electrode.
![]() | ||
Fig. 4 Confocal microscope images of CS-PVP composite films: (a) without CTAB addition and (b) with CTAB addition. |
The I(V) characteristics of the Al/CS-PVA/Au (Au ground) device are shown in Fig. 5. It can be observed that when the device voltage reaches ∼2 V (Vth) the device current starts increasing (OFF to ON transition), and at ∼3 V achieves a current value that is several orders of magnitude larger than that before Vth is achieved. At decreasing voltages the current level remains high (ON state). In further sweeps (not shown for figure clarity), the previous low current level of the OFF state is no longer observed, characterizing an irreversible OFF to ON transition.
To characterize the memory behaviour in the time domain,25 pulse trains of tp = 1 μs separated by 4 μs are applied. Fig. 6 shows the behaviour of the current in the device. When Vread = 1 V is established (A), the device is in the OFF state and a typical capacitor charging behaviour is observed as a current exponential decay. When Vwrite = 5 V is established (B) the current suddenly increases to the ON current and when Vread is restored (C) the current achieves a new current plateau (ON state current), which is also observed after further pulses. This behaviour indicates that 1 μs is sufficient time to consolidate the ON state in these devices.
It is interesting to observe the memory behaviour when shorter pulses of tp = 200 ns (shorter than tc and tp values reported for non-dispersed carbon spheres25) are applied, as shown in Fig. 7. As seen in Fig. 6, when Vread is applied, there is an exponential decay of the current. When a pulse with amplitude Vwrite is applied at 2.2 μs it is possible to see, in the first pulse, a transition behaviour similar to that shown in Fig. 3(c). After the first pulse, the ON state is consolidated since the current at Vread has already achieved steady state conditions for this voltage, which is repeated after further pulses.
![]() | ||
Fig. 7 Voltage drop measured in a resistor in series with the memory device during the application of a pulse train (tp = 200 ns). |
This characteristic is remarkable, since measurements performed in similar devices prepared without the addition of CTAB indicated a similar tt, but with tc = 10 μs (or even larger for other CS concentrations or CS types). We tested 20 devices applying pulse trains with tp = 1 μs and the ON condition was consolidated in all devices after the first pulse. 10 devices were then tested applying pulses with tp = 200 ns and also in this case all devices showed tc ≤ 200 ns. The devices were then tested 12 days after the ON state was written and all devices remained in this ON condition. This result indicates that in this memory device tt ≈ tc ≈ 200 ns and the application of pulses of tp = 1 μs encompasses a safety margin to reduce write operation failure. Applying 1 μs pulses a write circuitry can potentially write the ON state at a rate of the order of 1 Mbit s−1.
Further improvements in device performance are still necessary and the achievement of homogeneous carbon nanostructure dispersions certainly is a key issue here. The use of smaller carbon structures to allow the use of thinner composite films would also be beneficial for device size reduction. Determination of the impact of the agglomerate size on the device performance (ON/OFF current ratio, for example) also needs to be clarified, since agglomerates of controlled size and distribution may, to a certain extent, lead to performance enhancement or may even be required for the OFF to ON transition to occur.
This journal is © The Royal Society of Chemistry 2014 |