  Van der Waals encapsulation of two-dimensional materials within hexagonal
boron nitride (h-BN) stacks has proven to be a promising way to create
ultrahigh-performance electronic devices. However, contemporary approaches for
achieving van der Waals encapsulation, which involve artificial layer stacking
using mechanical transfer techniques, are difficult to control, prone to
contamination, and unscalable. Here, we report on the transfer-free direct
growth of high-quality graphene nanoribbons (GNRs) within h-BN stacks. The
as-grown embedded GNRs exhibit highly desirable features being ultralong (up to
0.25 mm), ultranarrow ( < 5 nm), and homochiral with zigzag edges. Our
atomistic simulations reveal that the mechanism underlying the embedded growth
involves ultralow GNR friction when sliding between AA'-stacked h-BN layers.
Using the grown structures, we demonstrate the transfer-free fabrication of
embedded GNR field-effect devices that exhibit excellent performance at room
temperature with mobilities of up to 4,600 $cm^{2} V^{-1} s^{-1}$ and on-off
ratios of up to $10^{6}$. This paves the way to the bottom-up fabrication of
high-performance electronic devices based on embedded layered materials.
