Salvador Eslava

Mesoscale assembly of chemically modified graphene into complex cellular networks

The widespread technological introduction of graphene beyond electronics rests on our ability to assemble this two-dimensional building block into three-dimensional structures for practical devices. To achieve this goal we need fabrication approaches that are able to provide an accurate control of chemistry and architecture from nano to macroscopic levels. Here, we describe a versatile technique to build ultralight (density ≥1 mg cm−3) cellular networks based on the use of soft templates and the controlled segregation of chemically modified graphene to liquid interfaces. These novel structures can be tuned for excellent conductivity; versatile mechanical response (elastic-brittle to elastomeric, reversible deformation, high energy absorption) and organic absorption capabilities (above 600 g per gram of material). The approach can be used to uncover the basic principles that will guide the design of practical devices that by combining unique mechanical and functional performance will generate new technological opportunities.

Single-source materials for metal-doped titanium oxide: Syntheses, structures, and properties of a series of heterometallic transition-metal titanium oxo cages

Titanium dioxide (TiO(2)) doped with transition-metal ions (M) has potentially broad applications in photocatalysis, photovoltaics, and photosensors. One approach to these materials is through controlled hydrolysis of well-defined transition-metal titanium oxo cage compounds. However, to date very few such cages have been unequivocally characterized, a situation which we have sought to address here with the development of a simple synthetic approach which allows the incorporation of a range of metal ions into titanium oxo cage arrangements. The solvothermal reactions of Ti(OEt)(4) with transition-metal dichlorides (M(II)Cl(2), M = Co, Zn, Fe, Cu) give the heterometallic transition-metal titanium oxo cages [Ti(4)O(OEt)(15)(MCl)] [M = Co (2), Zn (3), Fe (4), Cu (5)], having similar MTi(4)(μ(4)-O) structural arrangements involving ion pairing of [Ti(4)O(OEt)(15)](-) anion units with MCl(+) fragments. In the case of the reaction of MnCl(2), however, two Mn(II) ions are incorporated into this framework, giving the hexanuclear Mn(2)Ti(4)(μ(4)-O) cage [Ti(4)O(OEt)(15)(Mn(2)Cl(3))] (6) in which the MCl(+) fragments in 2-5 are replaced by a [ClMn(μ-Cl)MnCl](+) unit. Emphasizing that the nature of the heterometallic cage is dependent on the metal ion (M) present, the reaction of Ti(OEt)(4) with NiCl(2) gives [Ti(2)(OEt)(9)(NiCl)](2) (7), which has a dimeric Ni(μ-Cl)(2)Ni bridged arrangement arising from the association of [Ti(2)(OEt)(9)](-) ions with NiCl(+) units. The syntheses, solid-state structures, spectroscopic and magnetic properties of 2-7 are presented, a first step toward their applications as precursor materials.

Optical Property Changes in Low-k Films upon Ultraviolet-Assisted Curing

Ultraviolet-assisted curing (UV curing) has been applied recently to enhance the mechanical properties of low- k films. Knowledge about which UV energies are most effective is still limited and the consequences of applying the UV-curing process to integrated stacks in on-chip interconnects are unknown. To clarify these open questions, we investigated the optical properties of a SiCOH low- k layer by purged UV spectroscopic ellipsometry in the energy region 2-9 eV. The complex refractive index of the low- k film shows an absorption edge with a superimposed absorption band at 6.4 eV that vanishes upon UV-assisted curing. A comparison with Fourier transform infrared transmission demonstrates that the absorption at 6.4 eV must be attributed to the organic porogens, which also influence the absorption edge. Further analysis reveals the redshift of the absorption edge of silica-based low- k films with the presence of carbon species. The measured optical properties permit simulation of the standing wave pattern of light within the films in differently configured stacks.

Extending the Family of Titanium Heterometallic–oxo–alkoxy Cages

Here we investigate the synthesis of high-nuclearity heterometallic titanium oxo-alkoxy cages using the reactions of metal chlorides with [Ti(OEt)(4)] or the pre-formed homometallic titanium-oxo-alkoxy cage [Ti(7)O(4)(OEt)(20)] (A). The octanuclear Ti(7)Co(II) cage [Ti(7)CoO(5)(OEt)(19)Cl] (1) (whose low-yielding synthesis we reported earlier) can be made in better yield, reproducibly by the reaction of a mixture of heptanuclear [Ti(7)O(4)(OEt)(20)] (A) and [KOEt] with [Co(II)Cl(2)] in toluene. A alone reacts with [Co(II)Cl(2)] and [Fe(II)Cl(2)] to form [Ti(7)Co(II)O(5)(OEt)(18)Cl(2)] (2) and [Ti(7)Fe(II)O(5)(OEt)(18)Cl(2)] (3), respectively. Like 1, compounds 2 and 3 retain the original Ti(7) fragment of A and the II-oxidation state of the transition metal ions (Tm). In contrast, from the reaction of [Ti(OEt)(4)] with [Cr(II)Cl(2)] it is possible to isolate [Ti(3)Cr(V)O(OEt)(14)Cl] (4) in low yield, containing a Ti(3)Cr(V) core in which oxidation of Cr from the II to V oxidation state has occurred. Reaction of [Mo(V)Cl(5)] with [Ti(OEt)](4) in [EtOH] gives the Ti(8)Mo(V)(4) cage [{Ti(4)Mo(2)O(8)(OEt)(10)}(2)] (5). The single-crystal X-ray structures of the new cages 2, 3, 4, and 5 are reported. The results show that the size of the heterometallic cage formed can be influenced by the nuclearity of the precursor. In the case of 5, the presence of homometallic Mo-Mo bonding also appears to be a significant factor in the final structure.

Manipulation of the catalyst-support interactions for inducing nanotube forest growth

We show how an oxidative pretreatment of Fe, Co, or Ni growth catalyst on SiO2 support can be used to switch the growth mode of carbon nanotubes from tip growth to root growth, thus favoring the growth of dense, vertically aligned nanotube forests. The oxidative treatment creates a strong catalyst–support interaction at the catalyst–silica interface, which limits the surface diffusion and sintering of the catalyst nanoparticles and binds the catalyst to the SiO2 surface. This shows that the alignment and growth mode of nanotubes can be controlled, increasing the range of support materials giving dense nanotube forests.

Heterometallic cobalt(II)-titanium(IV) oxo cages; key building blocks for hybrid materials.

The two new heterometallic cobalt(II)/titanium oxo cages octanuclear [Ti(7)O(5)(OEt)(19)(CoCl)] (1) and pentanuclear [Ti(4)O(OEt)(15)(CoCl)] (2) have been obtained via condensation reactions of Ti(OEt)(4) in the presence of CoCl(2). This simple methodology gives access to key well-defined molecular building blocks for Co-doped hybrid materials.

Zeolite-inspired low-k dielectrics overcoming limitations of zeolite films

Spin-on zeolite films deposited from Silicalite-1 nanocrystal suspensions prepared by hydrothermal treatment of clear solutions have the required properties for insulating media in microelectronics. However, on the scale of the feature sizes in on-chip interconnects of a few tens of nanometers, their homogeneity is still insufficient. We discovered a way to overcome this problem by combining the advantages of the clear solution approach of Silicalite-1 synthesis with a sol-gel approach. A combination of tetraethyl orthosilicate and methyltrimethoxysilane silica sources was hydrolyzed and cocondensed in the presence of an aqueous tetraalkylammonium hydroxide template. The resulting suspension of nanoparticles of a few nanometers in size together with residual oligomeric silica species were spun onto support. The final zeolite-inspired low-k films (ZLK) with respect to pore size and homogeneity satisfied all requirements and presented excellent hydrophobicity, stiffness, and dielectric constant. The size and content of initially formed nanoparticles and the spatial hindrance promoted by occluded tetraalkylammonium molecules were found to be crucial elements in the definition of the final pore network.

Ultraviolet-Assisted Curing of Organosilicate Glass Low-k Dielectric by Excimer Lamps

A series of low- k films was exposed to UV-assisted curing (UV curing) with excimer lamps. The influence of the UV-curing wavelength, the UV-curing time, and the maximum pretreatment temperature were investigated. A mechanical, chemical, and optical characterization of this set of experiments is presented. It is revealed that the exposure to UV curing with 172 nm had sufficient energy to abundantly photodissociate Si-C H3 groups and to shrink the films. In turn, the elastic modulus was enhanced. As a side effect caused by the photodissociation of Si-CH groups, the content of Si-OH and Si-H moieties increased. Longer wavelengths (222 and 308 nm) showed less-drastic effects on low- k films because they do not provide sufficient energy to photodissociate Si-CH groups. This fundamental difference existing between different UV-curing wavelengths is evidenced by the optical characterization. Furthermore, this work also reveals the effect of pretreatment temperature on UV curing. Low pretreatment temperatures are required to keep enough photochemical reactivity and matrix mobility.

Electrical conduction of carbon nanotube forests through sub-nanometric films of alumina

We report both the growth of carbon nanotube forests and electrical conduction on W, Ti, and TiN substrates coated with an ultra-thin Al2O3 support layer. Varying the Al2O3 thickness, a good electrical contact and high nanotube density is possible for a 0.5 nm Al2O3 layer as such an ultra-thin film allows tunnelling. X-ray photoelectron spectroscopy shows that, when using these non-continuous Al2O3 films, Fe catalyst diffuses into the conducting substrates, eventually causing growth to stop. Forests grown on ultra-thin Al2O3 are potentially useful for applications as interconnects, supercapacitors, and heat spreaders.

Autonomous self-healing structural composites with bio-inspired design

Strong and tough natural composites such as bone, silk or nacre are often built from stiff blocks bound together using thin interfacial soft layers that can also provide sacrificial bonds for self-repair. Here we show that it is possible exploit this design in order to create self-healing structural composites by using thin supramolecular polymer interfaces between ceramic blocks. We have built model brick-and-mortar structures with ceramic contents above 95 vol% that exhibit strengths of the order of MPa (three orders of magnitude higher than the interfacial polymer) and fracture energies that are two orders of magnitude higher than those of the glass bricks. More importantly, these properties can be fully recovered after fracture without using external stimuli or delivering healing agents. This approach demonstrates a very promising route towards the design of strong, ideal self-healing materials able to self-repair repeatedly without degradation or external stimuli.